REPTILIAN AGE. JURASSIC PERIOD. 99 



resemblance to G. scitithim. Meek (Trans. Albany Inst. 1856), a Cretaceous species 

 from Vancouver's Island, but its truncated posterior margin is more oblique, and 

 its posterior umbonal slopes more angular. 



It seems to be more nearly related to the Oolitic species G. semicostatum, Lycett 

 (An. Nat. Hist. 1850), but is longer in proportion to its height, and has less dis^ 

 tinctly angular umbonal slopes. The specific name was given in honor of Dr. 

 George G. Shumard, formerly of the Geological Survey of Texas. 



Locality and position. — Southw^est base of the Black Hills, in Jurassic beds, asso- 

 ciated with Eumicrotis curta, Belemites densus, Grammatodon inornatus, &c. (No. 

 191.) 



Family ANATIOTD^. (See page 36.) 

 Genus MYACITES (Schlot.), Munster. 



Synon. — Myacites (part.), Scblot. Petref. 1820, 176. — Bronn, Leth. 1837, 174. — Munstek, in Goldp. Petref. Germ. 

 11, 1840, 259.— WooDWAKD, Man. Moll. 1850, 322.— Mohris and Lycett, Moll. Grt. Oolite, 1853, 111. 

 Panopxa (sp.), D'Okbigjiy, Palffiont. Fr. Ill, 1844, 329, and of various others (not Menard de la Groye, 



1809). 

 Pleuroviya, Agassiz, Etud. Crit. IV, 1845, 231.— Leowh. and Bkonn, Jahrb. 1846, p. 122. — Chesu, Man. 



Conch. II, 1862, 28. 

 Myopsis, Agassiz, Etud. Crit. IV, 1845, 251.— Chenu, Man. Conch. II, 1862, 28. 

 Etym. — f*u=f, a mussel. 

 Examp. — Myacites muscutoides, Schlot. 



Shell longitudinally ovate, oblong, or more or less elongate, very thin, nearly or 

 quite equivalve, without a defined lunule ; more or less gibbous in the central and 

 umbonal regions. Extremities gaping, the posterior side more than the anterior, 

 which is often nearly closed. Beaks moderately gibbous, placed between the 

 middle and the anterior extremity. Hinge probably always with one more or less 

 developed cardinal tooth in each valve; cardinal margin sometimes inflected, but 

 more generally erect, excepting near the beaks ; ligament external, short. Valves 

 often with a broad, undefined depression extending from the beaks to the basal or 

 antero-basal margin, usually deepening and widening as it descends. Surface with 

 concentric striae, and often more or less regular concentric ridges or costse, the whole 

 being, when well preserved, usually beset with minute granules. Muscular and 

 pallial impressions very faintly marked ; sinus of the latter broad and rounded. 



Animal unknown 



There is some confusion in regard to the limits of this genus, some authoi-s 

 including in it a wide range of forms evidently belonging to several genera, while 

 others restrict it to a few of these, or reject the name entirely, placing the species 

 in one or more of the allied groups. The name Myacites has perhaps met with less 

 general acceptance because it was not proposed by Schlotheim, who first used it, as 

 the name of a distinct genus, but to designate certain fossil shells supposed by him 

 to belong to the existing genus Mya. He merely added the termination ites in this 

 as in other instances, because the species he was figuring and describing were fossils, 

 and not because he supposed them to belong to a new genus. By examining his 

 work, it will be seen he wrote all the names of the genera to Avhich he referred his 



