A small piece of cartilage or bone, the element of Paaw, occurs in the tendon of the stapedius muscle in some extant marsupial and placental mammals. It has been nearly a century since the last comprehensive treatment of the distribution of the element of Paaw in mammals. The current report updates knowledge on this structure by synthesizing the subsequent literature and providing new observations of extant marsupials from the collections of the Section of Mammals, Carnegie Museum of Natural History, and two online resources for CT scanned data: DigiMorph.org and MorphoSource.org. We found an element of Paaw in some representatives of all seven extant marsupial orders: Didelphimorphia, Microbiotheria, Notoryctemorphia, Peramelemorphia, Paucituberculata, Dasyuromorphia, and Diprotodontia. In the first four orders, the element is substantial, longer than the long axis of the fenestra vestiuli (oval window), which holds the stapedial footplate; it is smaller than the long axis of the fenestra vestibuli in Paucituberculata and we do not have measures to report for the last two orders. In most marsupials examined, the element of Paaw contacts the petrosal behind the oval window, suggesting it functions as a sesamoid bone, increasing the lever arm of the stapedius muscle. Although there is some variability in the presence of the bone both between and within individual museum specimens, we interpret this as the result of preparation techniques rather than true variation.

To place the element of Paaw in its anatomical context, we describe in detail the ear region and middle-ear auditory apparatus of the gray four-eyed opossum, Philander opossum (Linnaeus, 1758), a didelphid from Central and South America, based on a CT scanned specimen from Carnegie Museum of Natural History. It has an ossified element of Paaw with a volume greater than the stapes. Comparisons are made with petrosals of Didelphis marsupialisLinnaeus, 1758, and Monodelphis domestica (Wagner, 1842), also based on CT scanned specimens.

Part VI is the final installment of a six-part series of papers discussing the fleas of Papua Province, Indonesia, and Papua New Guinea as a continued study of fleas in the Robert Traub flea collection deposited in the Carnegie Museum of Natural History, Pittsburgh, PA. Taxa from the genera Bibikovana, Geohollandia, and Hoogstraalia are included with emphasis on those species endemic to Papua Province, Indonesia, and Papua New Guinea. Four new species are described (Bibikovana engilisi, Bibikovana bosaviensis, Bibikovana mekiliensis, and Hoogstraalia nadchatrami). Bibikovana laciniosa bismarckensis is treated as a junior synonym of Bibikovana laciniosa laciniosa and Hoogstraalia novaeguineae is treated as a junior synonym of Hoogstraalia turdella. The females of Geohollandia solida and Hoogstraalia turdella are described and illustrated for the first time. The mammalian hosts of the three genera studied herein are summarized in Table 1. The avian host specificity of the genera Geohollandia and Hoogstraalia are substantiated with many new records of avian host associations included as Table 2. Geographic distributions of species of all three genera are summarized in Table 3. Keys for species of Bibikovana and Hoogstraalia and an updated key to the genera of Pygiopsyllomorpha are included. With the description of four new species and the proposed synonymies of two species, the total number of species in the superfamily Pygiopsylloidea in Papua Province, Indonesia, Papua New Guinea (including Bismarck Archipelago) and the Solomon Islands is 113. An additional eight species belonging to three other flea families (Ischnopsyllidae (3), Pulicidae (3), and Leptopsyllidae (2) bring the total number of flea taxa to 121 species (including subspecies).

Alabama has long been recognized as an aquatic biodiversity hotspot, and the Coosa River was home to over 80 endemic freshwater snail species. Due to human activity, over 40% of the snails have been extirpated, including the pleurocerid genus GyrotomaShuttleworth, 1845. Gyrotoma species varied in terms of shell shape and sculpture and were restricted to certain reaches of the Coosa River. Diversity estimates based on shell morphology have ranged from 44 nominal taxa to the modernly recognized six Gyrotoma species. However, basing pleurocerid species boundaries on qualitative morphological features poses many taxonomic and systematic issues. To better estimate diversity in the genus, geometric morphometrics and Gaussian mixture models were used to assign individual Gyrotoma shells to one of three clusters. Individuals in each cluster had significantly different shapes along with different combinations of quantifiable shell traits. No specific distributional patterns were observed between clusters. Though each cluster was not assigned to any specific taxonomic unit, morphometrics suggested a significant reduction in the number of Gyrotoma taxa. The clusters presented represent testable hypotheses of possible Gyrotoma diversity for when additional data are available.