CARBONIFEROIJS AGE. 23 



fine, obscure, regular, closely set radiating striae, about one liundred and fifty of which may be counted around the 

 border of large specimens, where eight or nine of them occupy the space of one line. 



Larger or dental valve depressed, having usually a broad, very shallow undefined mesial depression extending 

 from the front towards the beak ; cardinal margin armed with from eight to twelve oblique spines on each side of 

 the beak ; area of moderate breadth ; deltoid aperture very broad subtriangular, the upper angle being rounded 

 and the margins more or less projecting ; impressions of cardinal muscle subovate, diverging, attenuate above ; 

 adductor muscular scars small, narrow, subelliptical ; mesial ridge small, slightly prominent, and scarcely ever 

 reaching the middle of the valve. 



Dorsal or smaller valve following nearly the curve of the other; beak and central region concave ; ears flat ; area 

 well developed but narrower than that of the other valve, provided with mesial prominence, which, together with 

 the small bifid cardinal process projecting from its inner side, nearly or quite closes the foramen of the opposite valve. 

 From the base of this process there are extending on the inside of the valve five radiating ridges, two of which pass 

 obliquely outwards along the inner margins of the dental pits, while a third extends at right angles to the hinge a 

 little more than half way across towards the front of the valve ; the other two ridges are much shorter and occupy 

 intermediate positions between the central and lateral ridges, and are directed obliquely outward and forward. 

 Interior of both valves more or less granulose, the larger granules being arranged over a semicircular belt a little 

 within the border, which latter is occupied by very fine radiating granulose striae. 



Length, 0.62 inch ; breadth on hinge line, 1.13 inch. 



This shell is very closely allied to Chonetes SmifJin of Norwood & Pratten, and 

 may possibly prove to be only a variety of that species. It differs, hoAvever, in 

 being generally much larger, rather more compressed, and proportionally longer on 

 the hinge line ; its ears are also often much more extended and pointed than those 

 of G. Smithii. Another difference is that the coarser granules of the interior seem 

 never to be scattered over the central region of the valves as in Norwood & Pratten's 

 species. Again, the area of its smaller valve ranges more nearly at right angles to 

 the plane of the shell than in Illinois species. 



Locality and positiorb. — Near Fort Riley, Kansas Territory. Coal Measures. 

 (Type 1066.) 



Family STROPHOMEI^IDiE. 



Shell attached or free ; valves both convex, or one convex and the other 

 flat or concave ; hinge line straight, and provided with an area, which is 

 common to both valves, but usually wider in the ventral than the dorsal 

 valve ; arms without calcified supports, being probably fleshy and spirally 

 coiled ; shell structure fibrous only, or fibro-punctate. 



Animal unknown. 



This family includes OrtMs, Hemipronites, Klitambonites, Straphomena, Leptoena 

 and Tropidoleptus. Some authors also include in it the genera Chonetes and 

 Porambonites, but, as Mr. Davidson has demonstrated, the former belongs to the 

 Productidce ; while the affinities of the latter remain somewhat doubtful. 



This group presents one of the many interesting examples in the fossil world, of 

 an entire family, embracing several genera, and a great number of species, which, 

 after existing for immense periods of time, became entirely extinct, long before the 

 dawn of the present epoch. It is mainly a Palaeozoic family, since it appeared 

 almost with the beginning of life, and became wholly extinct at the close of the 

 Permian period, exce]3ting the genus Leptaina, which continued to be represented 

 by a few species until about the close of the Liassic period. 



