PLATE 41. 
Legend: dp. Deltaria. c. Crural bases. 
hp. Hinge-plate. b. Dental sockets. 
P. Delthyrium. sr. Crenulated ridge in sockets. 
p. Scar of pedicle-muscle. 
ATRYPINA, Hatt. 
Page 815. 
ATRYPINA DISPARILIS, Hall. 
Figs. 1, 2. Dorsal and ventral views of an average example. X 2. 
Niagara group. Waldron, Indiana. 
ATRYPINA IMBRICATA, Hall. 
Fig. 3. The interior of a pedicle-valve; showing the concave deltidial plates. 
X 2. 
Fig. 4, The interior of a brachial valve; showing the bilobed cardinal pro- 
cess, X 2. 
Fig. 5. Cardinal view of the same specimen. 24. 
Lower Helderberg group. Near Clarksville, N. Y. 
Artrypina Cririnront, Hall. 
Fig. 6. The interior of an incomplete brachial valve; showing the bilobed 
cardinal process and median muscular ridge. 3, 
Clinton group. Drift of Western New York. 
ATRYPA, Datman, 
Page 8'6. 
AtRYPA RETICULARIS, Linné. 
Fig. 7. Dorsal view of the youngest individual observed; showing the slight 
convexity of the brachial valve in the umbonal region, i‘s general 
depression anteriorly, low median sinus, few plications, erect beak 
and incipient deltaria. * 10. 
236 
