CLINTON, LOWER IIELDKRBEUG. C.UEMUXG, AND AVAVEHLY GROUPS. 457 



on each side of the carina, towards the base, a shallow depression. On some of the 

 specimens there are indications of very slight plications on the posterior side. 



Aperture oblique, broadly elliptical; peristome a lit- 

 tle sinuous at the carina, and sometimes slightly sinuous, ^ ' ^ f\\ 

 corresponding witli tlu' faint folds of the posterior side. / 



Surface marked l)v concentric lines and by broad uu- ""*"' ^ -* 



dulations, which sometimes give to portions of the shell s.?^. Trissj. *-^-i.__>i_>i 



a lobed appearance. 



A specimen of about the average size measures as follows : Height 12 mm. ; 

 length of the anterior side 25 mm. ; of the posterior 8 mm. ; width at base 22 mm. ; 

 thickness 18 mm. 



The characteristic feature of this species is the prominent carina or ele\ ation 

 along the dorsum. 



It may be distinguished from J'latijceras iuiequale of this formation by its some- 

 what larger size, the prominent carina, and the equal de^■eIopment of the right and 

 left side ; from Platijceras ihrsale by its smaller size, its Hattened form (a transverse 

 section being oval), and by the partial volution of the apex ; from Platijceras {Or- 

 thonychia) striatum by its arcuate form, the prominent carina and t!ie absence of con- 

 spicuous plications; from Platijceras carinatum by the equal development of the right 

 and left sides, and the absence of conspicuous plications ; from other species at 

 present known, by its small size, decidedly curved form, and prominent carina. 



Formation and locality. Chemung group, near Warren, "Warren county, Penn- 

 sylvania. 



PLATYCERAS STRIATUM! Simp-son, n. sp., Fig. 27. 



Shell subconical ; ape.v nol incurved or bent ; body essentially straight ; sides a 

 little curved from the base to the apex ; right and left sides equally developed ; width 

 at the base, length of anterior and posterior sides equal. Ou the anterior .><ide is a 

 narrow, angular, conspicuous carina, extending from viii^ /^Ik. ' >-^x z. 



the apex to the base ; on the posterior side are three 



oblique, broad, prominent, rounded ridges, with de- 

 pressions between them, which are wider than the 

 ridges. j 



Ajjcrture oval. The bases of all the specimens ^ , -•* 



observed arc attached to the rock so that the form of the peristome can not be defi- 

 nitely ascertained. 



A. 1'. >. — VOL. \vi. .3f. 



