108 Hill — Fossils of the Caprina Limesfone Beds. 



Upper Valve. — Xo satisfactory specimen of the smaller valve 

 has been found. In the specimen figured on plate XIII and in 

 accompanying figure at / there is a fragment preserved, super- 

 ficiall}^ resembling the shell of a Pecten, ^Yhich, however, u]>()n 

 careful inspection appears to be a broken remnant of tlie 

 smaller upper valve. It is composed of strongly radiating ribs 

 alternating with finer lines, and in the longitudinal section 

 shows a well-defined apophysis (/) corresponding to one of the 

 casts preserved in the larger valve («"). The section also shows 

 distinctly the casts of the two muscular aitophyses (e' e") of the 

 small valve, as in Iladiolites. 



The generic position of this form has been perplexing, and 

 possibly it deserves a distinct generic position. That it is 

 nearer Bncllolites than Hippurites is clearly shown l:\y the absence 

 of many of the characteristic features of the latter genus, such 

 as the numerous partitions which cross the interior cavit}^; the 

 different structure of the dwelling-chamber, and the presence of 

 only two instead of three longitudinal sutures. Upon the other 

 hand, it possesses many of the distinguishing characteristics of 

 Radioliteii, such as the prominence of the two well-defined Lon- 

 gitudinal sutures and the structure of the interior cavity. It 

 also differs from the genus SphcTndites, wliich is characterized 

 by one longitudinal suture. The cells are mostly rectangular, 

 while of the genus Badiolites, according to Zittel, there are five 

 or more sided. This distinction has been used by some writers 

 to make new genera, but the writer prefers to cTefer such action. 



The form occurs in great abundance in the limestones near 

 the water's edge of the Lampasas and I^eon creeks, in the eastern 

 suburbs of Belton, Texas, from whence the type specimens were 

 collected l)y Professor Wilson T. Davidson, in whose honor it is 

 named. It is also al)undant near Round Rock and Austin. 

 There is a possibility that this species may be the same as the 

 form entitled ''Hippiirites te.rnnus''^ of Roemer, figured and de- 

 scribed in Die Kriedebildungen von Texas, of Roemer ; but how- 

 ever strong the iriclination may be to think thc}^ are the same, 

 Ins descri])tions and figures are so radically dilfcrcnt that they 

 cannot be said to l)e identical. 



//. tc.niiius, as figured, sliows the cells to be polyginal instead 

 of rhomboidal, as in R. ducidsoDi, and the surface grooving and 

 cross-sections are entirely different in detail. 



R. davldsoni shows great variation in length, some of the forms 

 being very stunted and much thicker than the specimen figured. 



