PLATE XXVIII—Continued. 
Cybtina crassa, Hall. 
Fig-s. 13, 15. Two views of the extenior of a large individual. 
Corniferous limestone. Phelps, N. Y. 
Fig. 14. A cardinal view of a smaller individual, somewhat restored at the apex ; showing the imperforate 
dellidial covering and the irregular growth of the shell at the cardinal extremities. 
Corniferous limestone. Canandaigua, N. Y. 
Cyrtina umbonata, Htill, vtir. Alpenensis, vtir. iiov. 
Figs. 16, 17. Views of an individual of normal size at this locality, but much larger and more freely pli¬ 
cated than the Iowa forms of C. ztmbonata. 
Fig. 18. The conjoined valves split along the median septum; showing the extreme anterior extension of 
the latter, its acute anterior extremity, and the penetration of its median edge beyond the base 
of the dental lamelliE. 
Fig. 19. Lateral view ; showing the form of the spiral cone, its extension into the cavity divided by the me¬ 
dian septum, and the projection of the loop downward and toward the brachial valve. X 2^. (c.) 
Fig. 20. Fi'ont view of a preparation ; showing the normal shape of the spiral cones, which are somewhat 
constricted at their bases, expand for about one-half their lengih and thence taper very gradu¬ 
ally, terminating in blunt extremities. The form of the crura and loop is also shown. X H. (c.) 
Hamilton group. Alpena, Michigan. 
Cyrttna Hamiltonensis, Hall, var. recta, Hall. 
Figs. 21, 22. Views of a normal example ; showing the high, erect cardinal area. 
Hamilton group. Western New Yai’k. 
Cyrtina Hamiltonensis. Hall. 
Fig. 23. Axial section of the combined valves ; showing the form of the median septum and the supported 
dental plate. 
Fig. 24. An individual sectioned transversely at about the middle of the cardinal area ; showing the rela¬ 
tions of the deltidial covering, dental plates, median septum and spiral cones. 
Fig. 25. A cardinal view of an internal cast of conjoined valves. 
Fig. 26. The interior of a brachial valve ; showing cardinal process, socket-walls and muscular impression. 
Fig. 27. The exterior of a pedicle-valve with slightly distorted umbo. 
Fig. 28. Opposite side of the same specimen ; showing the elongate foramen on the deltidial plate. 
Fig. 29. An internal cast of the pedicle-valve ; showing the iiosition of the median septum. 
Fig. 30. A pi-ejiaration, showing the position of the spiral cones in the pedicle-valve. X 2. (c.) 
Hamilton grouji. Localities in Westei'n New York and Ontario. 
Fig. 31. Cardinal view of a specimen in which the spii als have been partially silicilied. X 2. 
Corniferous limestone. Falls of the Ohio. 
Fig. 32. An internal cast of the brachial valve ; showing the adductor scars. 
Fig. 33. Enlargement of the articulating apparatus of the brachial valve ; showing cai dinal process, den¬ 
tal sockets and socket-walls. 
Hamilton group. Westei-n New York. 
Fig. 43. An enlargement of the external surface ; showing the tine pustules. 
Fig. 45. An enlargement of the shell structure ; showing the punctse of various sizes, peneti'ating the fibi'ous 
layers. 
Fig. 46. A portion of a ti ansverse section of the umbonal region ; showing the deltidial covering, and the 
thickened dental lamellae at their union with the median septum, the edge of the latter extend¬ 
ing into the deltidial cavity. X 3. 
Fig. 53. The deltidial portion of the pedicle-valve enlarged; showing the elongate foraminal aperture and 
the fine cross striation of the deltidial covering. X 3. 
Hamilton group. Widder, Ontario. 
