CARBONIFEROTJS AGE. PERMIAN PERIOD. 59 



the teeth in the latter group. Externally, the differences between the Trigonias, 

 and the plicated, or costated species of Myoj)hor%a^ are about as great as those 

 between the latter and the smooth typical species of Schizodus. 



The genus Schizodus, as above defined, was probably introduced during the de- 

 position of the Upper Silurian rocks.^ It also occurs in the Devonian,^ and is 

 most common in the Carboniferous and Permian deposits. If any of the species 

 included in the genus Myophoria, by Bronn, belong to this group, it must have been 

 represented 9.uring the Triassic epoch. 



Schizodus oTatus. 



(Plate II, Fig. 11, a, 6.) 



Axinus {Schizodus) ovatus, Meek & Hayden, Proceed. Acad. Nat. Scl. Phila. Deo. 1858, 262. 

 Comp. Schizodus iruncaius, King, Permian Fossils England, p. 193, pi. xv, fig. 25 to 29, inclusive. 



Shell longitudinally ovate, moderately convex, the most gibbous portion of the valves being slightly in advance 

 of the middle. Anterior side broader than the other, but somewhat narrowly rounded ; posterior side more con- 

 tracted — compressed and obliquely truncated above, and subangular below ; base forming a regular semi-ovate 

 curve, the most prominent part of which is in advance of the middle. Cardinal margin short, straight, and inter- 

 secting the obliquely truncated posterior edge at an angle of about 130°. Beaks rather elevated, incurved at right 

 angles to the hinge, and placed a little in front of the middle ; posterior umbonal slopes prominently rounded, or 

 subangular from the beaks to the postero-basal extremity. (Surface, muscular, and pallial impressions unknown.) 



Length, 0.65 inch; height, 0.45 inch; convexity, 0.32 inch. 



This species is very closely related to S. truncatus of King, and may possibly 

 prove to be identical, when a direct comparison of specimens can be made. It is 

 more nearly like his fig. 27, pi. xv (Permian Fossils) than any of the other varieties 

 represented by him, but differs in being more sharply rounded in front, while its 

 beaks are a little more depressed. Its posterior umbonal slopes appear also to be 

 less prominent than in any of his figures of S. truncatus. 



Locality and position. — South Cotton-Wood Creek, Kansas ; in Permian mag- 

 nesian limestone. It also occurs there in lower beds containing many Coal Measure 

 species. (Type 3960.) 



Family N^UCULANID.^. 



Shell longer than high, subovate, oblong or subelliptical, equivalve, 

 usually somewhat pearly within ; hinge provided with small interlocking 

 cardinal plates or denticles, as in the Nuculidm ; ligament internal or 

 external ; margins of valves smooth within ; pallial line more or less 

 sinuous. 



Animal with mantle margins open, fringed, and usually provided with 

 ventral lobes ; labial palpi very long, convoluted ; siphons rather long, 

 slender, partly united and retractile ; gills plumose, attached throughout 

 their length. Foot deeply grooved, geniculate, and usually with serrated 

 margins. 



* Anodoniopsis securiformis, of McCoy, is apparently an Upper Silurian example of this genus. 

 " Murchison, Verneuil, and Keyserling refer to this genus, a species (S. devonicus) from the Devo- 

 nian Rocks of Russia. 



