108 Hill Fossils of the Caprina Limestone Beds. 



l'l>l><'i' Vlr<\ 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- 

 ficially resembling the shell of a Pecten, which, however, upon 

 careful inspection appears to be a broken remnant of the 

 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 (/') ^ ne section also shows 

 distinctly the casts of the two muscular apophyses (Y e") of the 

 small valve, as in Radiolites. 



The generic position of this form has been perplexing, and 

 possibly it deserves a distinct generic position. That it is 

 nearer Radiolites than Hippurites is clearly shown by the absence 

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

 as the numerous partitions which cross the interior cavity ; the 

 different structure of the dw r elling-chamber, and the presence of 

 only two instead of three longitudinal sutures. Upon the other 

 hand, it possesses many of the distinguishing characteristics of 

 Radiolites, 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 Sphxrulites, w r hich is characterized 

 by one longitudinal suture. The cells are mostly rectangular, 

 while of the'genus Radiolites, 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 defer such action. 



The form occurs in great abundance in the limestones near 

 the water's edge of the Lampasas and Leon creeks, in the eastern 

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

 collected by Professor Wilson T. Davidson, in whose honor it is 

 named. It is also abundant near Round Rock and Austin. 

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

 form entitled "Hippurites lexanus" of Roomer, figured and de- 

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

 ever strong the inclination may be to think they are the same, 

 his descriptions and figures arc so radically different that they 

 cannot be said to be identical. 



If. tern-mis, as figured, shows the cells to be polygonal instead 

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

 cross-sections are entirely different in detail. 



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

 being very stunted and much thicker than the specimen figured. 



