KKl'TILIAN AGE. JURASSIC PERIOD. 



77 



nr divaricating stria' mi /'. fW/<f/<////)/x. of Sowerby, which to the unassisted eye 



seems ti) l>c oiil\ marked with concentric lines. It is likewise even possible, we 

 tliink, that tliis genus may he found to include some ( 'retaceoiis species with straight, 

 rijjid, radiating costa', such as /'. /.i///. /,/,./ and /'. r<il<>iii<ii/rn9i*, D'Orbigny, for on 

 both of these shells, which are ornamented with straight, radiating costa'. \\e observe 

 an entirely distinct s\stem of curved, radiating, or divaricating stria;, which on the 

 lateral margins cross the costs: ol>li(|iiely ; \\liile tliese shells have the form and deep 

 h\-^al sinus of the typical ( 'niii/>/<>,,,</, *. Still, they may present some differences 

 in the nature of the hinge or interior that, along with their surface markings, 

 would place them in ii distinct genus. 



The topical species of this genus will be at once known from all the other groups 



of I'n-i'u, i,/,i. by their peculiar ornamentation alone. That these and various other 



fossil and recent types usually referred to the genus .fteten, should be placed in 



different genera from that group, as t\ pitied by the recent P. Jacobins, P. maximum, 



is maintained by Prof. Agassi/, cannot be reasonably doubted. 



This genus was introduced during the Jurassic epoch, and ranges through several 

 members of the Cretaceous system, the deposition of which it seems not to have 

 survived. 



t'amptoncctes belliatriatiis. 



Peelen bellittriatut, MKKK, Proceed. Acad. Nat. Sci. 1'hila. July, 1860, 311. 

 Cami>ionrcta bellutriatui, MEEK, Smithsonian Cat. N. Am. Jurassic Fossils, 1864. 



Shell very thin, compressed-lenticular, snborbicnlar ; valves nearly equally convex ; binge line equalling two-fifths 

 to one-half the transverse diameter of the valves ; posterior ear very short or nearly obsolete, flat, and obliquely 

 truncated ; anterior ear larger, flattened, and marked by rather distinct lines of growth in the right valve separated 

 from the adjacent margin by a more or less angular sinus one-third to one-half as deep as the length of the ear, 

 measuring from the beak. Surface strin very fine, regular, sharply impressed, and increasing In number by the 

 intercalation of others between as they diverge in extending from the umboual region so strongly arched as to run 

 out on the hinge lino near the beaks ; concentric stria* fine, regular, closely arranged, and often nearly orquite obsolete 

 on the flattened spawn between the impressed radiating strisj, to which latter they impart a sub-punctate appearance. 



Length of a large shell, 2.65 inches ; breadth from hinge to ventral margin, 2.26 inches ; convexity, 0.64 inch. 



A DC 



Camptoneete* bellistriatu*. 



A. OuUide view of a small lefl valve. D. Enlargement of surface stri of MUM. B. Inside view of a Urge right 

 valve [the appearance of radiating markings within is an error in the engraving]. C. Outline of right and left 

 valves united. 



