ACT.] 



CEPHALOPODA. 



431 



Each septum began to form at the circumference of the shell, and slowly approached 

 the siph uncle as the animal moved forward in the body chamber. The siph uncle, 

 being a point of muscular attachment, was not vacated by the animal between any 

 two septa until the anterior one had been firmly closed by attachment to the 

 siphuncle, forming a chamber of support. 



The fossil shells are very thin in proportion to their size. They are not po- 

 rous, like those of the Brachiopoda ; nor horny, like the Crustacea ; nor of the same 

 composition as the Gasteropoda or Lamellibranchiata. Generally the exterior shell 

 is destroyed, even when the associated shells of other classes are well preserved. 

 Sometimes the shell appears as if it had melted and run together, or run down 

 upon the siphuncle. Such molecular change will occur in one part of a specimen 

 while another part is unchanged. The general form of the shell is of family im- 

 portance. The shape of the siphuncle and the external markings are of generic 

 importance. We recognize the following families : 



FAMILY ASCOCERATID^. Ascoceras. 



FAMILY CYRTOCERATID^E. Cyrtoceras, Cyrtocerina, Oncoceras. 



FAMILY DISCOSORID^E. Discosorus. 



FAMILY ENDOCERATID^E. Cameroceras, Colpoceras, Endoceras. 



FAMILY GOMPHOCER ATID^E. Gomphoceras. 



FAMILY GONIATITID.E. Goniatites. 



FAMILY GYROCERATID^E. Gyroceras. 



FAMILY LITUITID^:. Lituites. 



FAMILY NAUTILID^E. Discites, Nautilus, Pteronautilus, Solenochilus, Temno- 



chilus, Trematodiscus. 

 FAMILY ORTHOCERATIDJE. Actinoceras, Bactrites, Gonioceras, Huronia, Or- 



moceras, Orthoceras, Trematoceras. 



FAMILY PHRAGMOCERATID^E. Phragmoceras, Streptoceras. 

 FAMILY PILOCERATID.E. Piloceras. 

 FAMILY TROCHOCERATID^E. Trochoceras. 

 FAMIL y TROOHOLITID^:. Trocholites. 

 FAMILY UNCERTAIN. Petalichnus, Sserichnites, Teratichnus, Trachomatichnus. 



ACTINOCERAS, Bronn, 1837, Letbaea Geognos- 

 tica, p. 97. [Ety. aktin, ray ; keras, 

 horn.] Exterior like Orthoceras; si- 

 phuncle very large, inflated between 

 the chambers, and connected with a 

 slender central tube by radiating plates. 

 Type A. bigsbyi, A. richardsoni, and A. 

 lyoni. The genus was established be- 

 fore the species were defined. 



beaudanti, Castelnau, 1843, Syste'me Silu- 

 rien, p. 31. Not recognized. 



beaumonti, Castelnau, 1843, Systeme Silu- 

 rien, p. 32. Not recognized. 



bigsbyi, Stokes, 1840, Trans. Geo. Soc., 2d 

 series, vol. 5, p. 707, Chazy Gr. 



blainvillei, Castelnau, 1843, Systfeme Silu- 

 rien, p. 31. Not recognized. 



cordieri, Castelnau, 1843, Systeme Silurien, 

 p. 31. Not recognized. 



deshayesi, Castelnau, 1843, Systeme Silu- 

 rien, p. 32. Not recognized. 



dufresnoyi, Castelnau, 1843, Systeme Silu- 

 rien, p. 32. Not recognized, 

 inops, Daw- 

 son, 1868, 



A c a d . 



Geol. p. 



314, Carb. 

 lyoni, Stokes, 



1840, 



Trans. 



Geol. Soc.. 



vol. 5, p. 



707, Black 



Riv. Gr. 

 richardsoni, 



Stokes, 



1840, FlG 725. Actinoceras rich- 



Trans. ardsoni. 



Geol. Soc., 



2d series, vol. 5, p. 708, Black Riv. Gr. 

 simmsi, Stokes, 1840, Trans. Geo. Soc.. 2d 



series, vol. 5, p. 708, Sil. 



