LOWER SILURIAN. 203 



323, Tceniaster spinosa B. ; Fig. 324, the Crinid Lecanocrinus ehgans B. ; Comarocystites 

 Shumardi M. & W., from Missouri; Fig. 325, the two-armed Cystid Pleurocystis squa- 

 mosum B., of the Trenton, in Ottawa, Canada; also, Agelacrinites Billingsii, Chapman. 



The number of Cystids described by E. Billings from the Lower Silurian of Canada 

 is 21; making in all, for this era in North America, thus far known, 22; the Crinids of 

 the same era amount to 50 species, and the Star-fishes to 11 ; 13 of the Crinids and 

 8 of the Star-fishes are Trenton species. 



3. Mollusks — (a.) Bryozoans. — Species of Retepora and Ptilodictya (related to 

 Figs. 306, 307) are common; Clathrqpora fiabellata H. 



(b.) Brachwpods. — Figs. 326, 327, Orthis lynx Eich.; 328, 0. occidentalis H. ; 329, 

 0. testudinaria Dalm. ; 330, 0. tricenaria Con. ; 331, Leptcena sericea Sow. ; 332, Stro- 

 phomena ruyosa H. (formerly Leptcena depressa Sow. ; 333, Stroph. alternata Con. ; 334- 

 336, Rhynchonella capax Con. ; 337, 338, Rhynchonella (?) bisulcata Emm. ; 339, Obolus 

 filosus ( Orbicula ? filosa H.); 340, Lingula quadrata H., and other LingulellcR ; species of 

 Discina, Trematis, Camerella, etc. 



(c.) Lamellibranchs. — Fig. 341, Avicula (?) Trentonensis Con.; 342, Ambonychia belli- 

 striata H. ; 343, Tellinomya nasuta H.; also Conocardium immaturum B., of Black 

 River limestone, Ottawa; species of Modiolopsis, Cyrtodonta. 



(d.) Gasteropods. —Fig. 344, Pleurotomaria lenticularis Con., very common in the 

 Trenton; also several other species of the genus; 345, Murchisonia bicincta McCoy; 

 346, M. bellicincta H., often four inches. long; 347, Helicotoma planulata Salter, from 

 Canada; Ophileta Owenana, M. & W., from the Galena limestone; 348, Bellerqphon 

 bUobatus Sow. — very common; 349, same, side-view; 350, Cyrtolites compressus H.; 

 351, 352, Cyrtolites (?) Trentonensis H. The genus Cyrtolites is like a partly uncoiled 

 Bellerophon, and is not chambered. The genera Bellerophon and Cyrtolites are sup- 

 posed to belong to the group of Heteropods. There are also several Patella-like species 

 of Metopfoma (formerly Capulus and Patella), a genus which began in the Calciferous 

 beds; species of Holopea, Cyclonema, Trochonema, JEunema, Raphistoma, Subulites, 

 etc. Maclurea magna, a Chazy species (Fig. 311, p. 191), occurs in the Trenton, in 

 Middle Tennessee (Safford); Chiton Canadensis B. occurs in the Black River limestone, 

 in Ottawa. 



(e.) Pteropods. — Fig. 368 represents Conularia Trentonensis H., a delicate four-sided 

 pyramid, apparently admitting of some motion at the angles, but 

 having septa within in the smaller extremity (a); it is supposed 

 therefore to be the shell of a Pteropod by Barrande ; b is an en- 

 larged view of the surface. 



(f.) Cephalopods. — Fig. 353, Orthoceras junceumll., a small 

 Trenton species; 354, 0. vertebrate H., also Trenton, the figure 

 reduced to one-third; 355, part of an Ormoceras tenuifilum H., 

 common in the Black River limestone, and sometimes over two 

 feet long: the genus Ormocera* is peculiar in the beaded form of 

 the siphuncle. Other common species of the Orthoceras family 

 are the Endoceras proteiforme H., and the Gonioceras anceps H. 

 The Endoceras was in some cases fifteen feet long, and nearly one 

 foot through. In this genus (named from the Greek /cepas, horn, 

 and evSov, within), there is a concentric structure of cone within nu n 

 cone. In Gonioceras, the partitions are much crowded and have a double curvature, 

 and the siphuncle is central. 



Among the curved species, Fig. 356 is Cyrtoceras annulatum H. ; a, a transverse 

 section; Fig. 357, Cryptoceras undatum (Lituites undatus H.), abundant in the Black 

 River limestone; Fig. 358, Trocholites Ammonius Con., of the Trenton; 358 a, trans- 

 verse section. In Cryptoceras, the spiral is open at the outer extremity, and the 

 siphuncle is dorsal; while, in Trocholites, it is closed and tightly coiled throughout. 

 Lituites, which first appeared in the Calciferous, differs from Cryptoceras in having 

 the siphuncle sub-central. The genus Phragmoceras has the mouth of the shell very 

 »uch contracted, by a bending inward of the sides; P. immaturum B., is from the 

 Black River limestone of Canada. 



