n.ATt; XXVIU— Coullnaod. 



Cyktina ckassa, Hull. 



Flga. 18, 16. Two views of the exterior of a large individual. 



Coniifei-ous limestone. Plielps, N. T. 

 ¥ig. 14. A cardinal view of a xnialler individual, somewhat restored at the apex ; showing: the imperforate 

 dellidial coverinff and the irregular growth of the shell at the cai-dinal extremities. 



Comiferous limestone. Canandaigua, N. Y. 



Cyktina umbonata, Ilall, var. Ai.penkxsis. var. nov. 



Figs. 16, 17. Views of an individual of normal size at this locality, but much lai-ger and more freely pli- 

 cated than the Iowa forms of 0. uinbonata. 



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. 



Kig. 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. Front view of a preparation ; showing the normal shaj^e of the spiral cones, which are somewhat 

 constricted at their bases, expand for about one-half their length and thence taper very gradu- 

 ally, terminating in blunt extremities. The form of the crura and loop is also shown. X li. (o.) • 

 Hamilton group. Alpena, Michigan. 



Cyktina Hamiltonensis, Hall, var. kecta, Hall. 



Figs. 21, 22. Views of a noi-mal example ; showing the high, erect cardinal area. 

 Hamilton group. Western New York. 



Cyktina Hamiltonensis, Hull. 



Fig. 23. Axial section of the combmed valves ; showing the foi-m of the median septum and the suppoi-ted 



dental plate. 

 Fig. 24. An individual sectioned transversely at about the middle of the cai-dinal ai-ea ; 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, soclcet-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 position of the median septum. 

 Fig. 30. A preparation, showing the position of the spiral cones in the pedicle-valve. X 2. (c.) 



Hamilton group. Localities in IVestei-n New York and Ontario. 

 Pig. 31. Cardinal view of a specimen in which the spiials have been partially silicified. X 2. 

 Corniferous limestone. Falls of the Ohio. 

 An internal cast of the Itracbial valve ; showing the adductor scai-s. 



Enlargement of the articulating apparatus of the bi'achial valve ; showing cardinal process, den- 

 tal sockets and socket-walls. 



Hamilton gi-oup. Western New York. 

 An enlargement of the external surface ; showing the 6ne pustules. 

 An enlargement of the shell structure ; showing the puncla; of various sizes, penetrating the fibrous 



layers. 

 A portion of a transverse 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. 

 68. The deltidial portion of the pedicle-valve enlarged ; showing the elongate foraminal apertui'e and 

 the fine cross striation of the deltidial covering. X 3. 

 Hamilton group. Widder, Ontario. 



