TKENTON LIMESTONE. 117 



155. 2. ORTHIS TESTUDINARIA. 



Pl. XXXII. Figs. la-/. 



Orthis testudinaria. D.ilman, Vet. Acad. Hand. Stockholm, 1827, p. 115, t. 2, f. 4 a, b, c, d, e. 

 Trigonotrctia testudinaria. Bronn, Leth. Geognostica, pag. 82, tab. 3, fig. 2. 

 Orthis testudinaria. Hisinger, I'ct. Suecica, 1S37, pag. 71, lab. 20, fig. 11 a, b, c. 



— — 7 SowERBY in Sil. System, 1S39, pag. 640, pi. 20, fig. 9. 



— — CoNR.\D, Ann. Geol. Report of 1839, p. 63 ; 1S40, p. 201 ; 1841, p. 37. 

 Orthis striatula, Conb.vd in MS. Emmond, Geol. Report, 1842, pag. 394, ill. 105, fig. 3. 

 Orthis testudinaria 7 Emmons, lb. pag. 40 1, fig. 4. 



Suborbicular, plano-convex ; cardinal line straight, shorter than the widtli of the shell ; 

 dorsal valve convex, much elevated towards the beak, often with an elevated ridge down 

 the centre; beak small, slightly incurved; ventral valve (lat, or with a longitudinal 

 depression along the centre, wliich often produces an emargination in front ; cardinal area 

 small ; foramen small, triangular ; surface covered with fine stria;, whicli bifurcate towards 

 the margin, and are crossed by elevated threadlike lines, giving them a crenulated 

 appearance. 



This species is variable in some respects. Nearly all the specimens from New-York have 

 a regularly depressed-convex dorsal valve, with a flat or sometimes slightly convex ventral 

 valve ; while those from Ohio, wliich are specifically identical, have very frequently an 

 elevated longitudinal ridge on the dorsal valve, and a depression along the flat or ventral 

 valve. These characters do sometimes occur in the Trenton specimens, but not often. In 

 the western specimens the striae are usually more prominent than in those of New- York, 

 and the shells likewise attain a larger size. 



This species is exceedingly abundant in the Trenton limestone, being, with the Lcptama 

 sericea, the two most abundant fossils in the rock. It often covers entire surfaces of the 

 thin strata, both as perfect specimens and as separated valves. It appears to be in equal 

 abundance in nearly all the western localities of the same rock. 



A comparison of a Swedish specimen of Orthis testudinaria with those of New-York, 

 shows no essential difference ; the former being a little more elongated, and the ventral 

 valve more convex than in the prevailing forms of the Trenton limestone. 



Fig. 1 a. Ventral valve, showing the beak of the dorsal valve and the foramen. 



Fig. 1 b. Profile view of the same shell, showing the convexity of the dorsal valve : the same is shown 

 in the cardinal view, 1 d. 



Fig. 1 e. View of the dorsal valve, showing the small projecting beak. 



Fig. 1 e. A smaller specimen of the same species. 



Fig. 1 / The interior of the dorsal valve, showing the toothhke processes margining the foramen, and 

 projecting forwards into the shell. 



Fig. 1 g. The interior of the ventral valve, showing the visceral impression, and the two lateral tooth- 

 like processes which imite with the processes of the other valve. 



Fig. 1 /;. Cast of the dorsal valve, showing the two visceral impressions, one on either side of a central 

 elevated ridge. 



Figs. 1 i, k, I. The same species, from the Blue hmestone of Ohio. 



