REPTILIAN AGE. JURASSIC PERIOD. 69 



In the dorsal valve, there is no peduncular attachment, but the scar of the 

 divaricator muscles is located nearly as in the other valve. The two anterior 

 occlusor impressions of this valve are placed in contact, centrally, side by side, at 

 the farthest anterior extremity of the visceral area ; and just behind these, and a 

 little separated from each other, are the two impressions of the posterior occlusor 

 muscles. About midway between the latter and the posterior extremity of the 

 visceral area are situated, near each lateral margin, the small scars of the posterior, 

 external, and central adjuster muscles, of which there are four on one side, and 

 three on the other, one of the latter being considerably larger than the others. 



This genus is closely allied to the recently separated Lingulepis, and until the 

 muscular impressions of the type of that proposed genus have been more clearly 

 determined, there is some room for doubt whether or not it is really distinct. The 

 principal differences yet observed between these two types, consist in the ovate 

 subtrigonal form, and more attenuate beak in Lingulepis ; and the much more 

 distinctly trilobate visceral scar of its ventral valve. The visceral scar of its dorsal 

 valve is also more flabelliform than in the typical Lingulas. 



The genus Lingibla was introduced at a very early period ; at any rate, we find 

 species in no way distinguishable from it, at least by any external characters, in 

 some of the oldest of the Silurian rocks. Its remains are likewise found ranging 

 through aU subsequent formations, and several species are known to inhabit our 

 existing seas. It seems to have attained its maximum development during the 

 Silurian Age. The living species are foimd on the coast of California, the Sand- 

 wich and Philippine Islands, and on the shores of South and North Carolina, and 

 the West Indian Island of St. Thomas. They inhabit shallow water, being gene- 

 rally found at low tide, with their long peduncle deeply penetrating the sand or mud. 



liingnla breTirostris. 



(Plate III, Fig. 3, a, 5.) 



Lingula brevirostris, Meek & Haydes, Proceed. Acad. Nat. Sci. Phila. March, 1858, p. 50 ; Id. lb. Meek & Hatdek, 

 Oct. 1860, p. 419. 



Shell subovate, varying to spatulate-ovate, very thin ; lateral margins nearly straight, or somewhat convex, 

 generally converging a little towards the beaks ; front subtruncate or abruptly rounded ; cardinal extremity 

 narrowly rounded. Beaks obtuse and apparently not extending beyond the cardinal margin. Valves equally 

 convex along the middle, and compressed towards the front and sides. Surface polished, and marked by fine, 

 rather obscure lines of growth ; on the surfaces of the inner laminae traces of extremely fine longitudinal stri^ 

 are sometimes visible by the aid of a magnifier. 



Length, 0.57 to 0.58 inch; breadth, 0.33 to 0.35 inch; convexity of the two valves, 0.16 inch. 



This shell bears some resemblance to the Oolitic species L. Beanii, Phillips, but 

 is generally broader toward the front, and more obtusely rounded at the cardinal 

 extremity. 



Locality and position. — Southwest base of the Black Hills, Idaho Territory — 

 lower part of the Jurassic rocks of that region. (Type 206.) 



