LOWER SILURIAN. 203 



323, Tamiaster spinosa B. ; Fig. 324, the Crinid Lecanocrinus elegans B. ; Comarocystites 

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

 mosum B., of the Trenton, in Ottawa, Canada; also, Agelacnnites Billinysii, 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; Clathropora flabellata H. 



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

 0. testudinaria Dalm. ; 330, 0. tricenaria Con.; 331, Leptm?ia sericea Sow.; 332, Stro- 

 phomena rugosa H. (formerly Leptcena depressa Sow.; 333, Stroph. alternate/, Con.; 334- 

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

 filosus ( Orbicula ? Jilosa H.); 340, Lingula quadrata H., and other Lingulelke ; 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. bellicineta H., often four inches long; 347, Helicotoma planulata Salter, from 

 Canada; OphUeta Owenana, M. & W., from the Galena limestone; 348, Bdhrophon 

 bilobatus 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 Belle rophon and Cyrtolites are sup- 

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

 of Metoptoma (formerly Capidus and Patella), a genus which began in the Calciferous 

 beds; species of Holopea, Cyclonema, Troclwnema, Eunema, Rapliistoma, 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 («); it is supposed 

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

 larged view of the surface. 



[f.) Cepihalopods. — Fig. 353, Orthoceras junceumH., a small 



Trenton species; 354, O. vertebrale 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 Ormoceras 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 ^t W ^ty^ 



foot through. In this genus (named from the Greek xepa?, horn, 



i - !• VI- n it. • ,. i • c vi • Conularia Trentonensis. 



and evSov, intnin), there is a concentric structure ot cone within 



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 annidatum H. ; a, a transverse 

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

 River limestone; Fig. 358, Trocholite.s 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. 

 Litnites, 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 



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



Black River limestone of Canada. 



