REPTILIAN AGE. JURASSIC PERIOD. 101 



though it evidently never had radiating costae, like -we see on well-marked species 

 of Plioladomya, as defined by most authors. Its true generic relations, however, 

 must remain doubtful until better specimens can be obtained. 



Locality and jposition. — Southwest base of the Black Hills, near the lower part 

 of the Jurassic series of that region. 



Myacltes isubellipticns. 



(PlATE IV, Fig. 6, a, b, c.) 

 Panopsra {Mijacites) subeUiptica, Meek & HAYDEif, Proceed. Acad. Nat. Soi. Phila. March, 1858, 52 ; 

 ib. Oct. 1860, 418. 

 Shell narrow, elliptical, or subovate, moderately convex ; extremities rather narrowly rounded, the posterior end 

 being more compressed than the other ; base forming a very broad semi-elliptic curve ; beaks located in advance 

 of the middle, rather depressed, the right one sometimes a little more elevated than the other ; surface of cast 

 marked by small irregular wrinkles of growth ; hinge and muscular and pallial impressions unknown. 

 Length, 2.03 inches ; height, 1.09 inch ; breadth, 0.70 inch. 



In its general appearance this species resembles Panopcea 2>ereg7"ina, D'Orbigny, 

 as figured by Murchison, de Verneuil and Keyserling, in their Geol. Euss. II, part 3, 

 pi. xl, fig. 10, but it is proportionally a little shorter, narrower posteriorly, and more 

 convex in the antero-ventral region, while its extremities appear to be less gaping. 



It is even more nearly similar in form to P. Neocomiensis, Lehm. sp., from the 

 Lower Green Sand of the Old World, but diff'ers from most of the figures we have 

 seen of that species, in being more narrowly rounded at the extremities, and in 

 having less prominent beaks. 



Locality and position. — Southwest base of the Black HUls, near the lower part 

 of the Jurassic series of that region. (Type No. 200.) • 



Genus THRACIA, Leach. 



Synon.—Thracia, Leach, MSS. 1819; Blainville, Diet. Sci. Nat. XXXII, 1824, 347; and Malac. 1825, 564.— Rang, 

 Man. 1829, 324.— Deshayes, Encyc. Meth. Ill, 1830, tab. 1832; ib. p. 1038; and in Lamk. 2d ed. VI, 

 1835, 82.— Menke, Syn. 2d ed. 1830, 119, &c. Not Thracia, Westwood, 1840 (gen. Insects). 

 Odoncinetus, Da Costa, Cat. Syst. 1829, 32. 

 Odontocineta (Corr.), Agassiz, Index Universalis, 1846, 255. 

 Etym. — Sfiaitiof, Thracian ? 

 Type. — Mya pubescens, Penn. 



Shell longitudinally oblong or ovate, inequivalve, rather thin; posterior side more 

 or less contracted, compressed, and gaping; surface concentrically striate, sometimes 

 minutely scabrous. Hinge consisting of a thick, slightly prominent cartUage pro- 

 cess in each valve, with a free crescentic ossicle in front. Ligament partly internal. 

 Pallial sinus rather shallow; muscular impressions small. Outer shell layer con- 

 sisting of distinct nucleated cells. 



The genus Thracia was probably introduced during the deposition of the Liassic 

 or oldest Jurassic rocks. It also ranges through the later formations, and appears 

 to attain its greatest development in our existing seas. The species, however, were 

 quite numerous at several intermediate periods, particularly during the deposition 

 of the Neocomian rocks. 



We observe several authors place Itupicola, of Bellevue, 1802, as a subgenus 



