406 OARBO.flFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



break or interruption, but, on the contrary, variety passes into variety In' the easiest 

 griidation.s. 



Youn}^ specimens are often neai'ly circular in shape, with the elements of fold 

 and sinus almost imperceptible, and from certain localities such specimens only have 

 come to hand. They sometimes assume much the aspect of the latei' forms of devel- 

 opment of the genus CleiotJiyris. 1 have identified Cleiothyru among the fossils 

 examined in the case of only two specimens, but possibly among the shells just men- 

 tioned may occur representatives of this genus which their superficiallj^ exfoliated 

 condition has prevented mj^ recognizing as such. 



Kej'es and Schuchert have used for this species the specific term argentea, pro- 

 posed by Shepard fourteen j^ears before Hall's Terebratula siibtilita was described. 

 Relegating, for the time, the question whether Terebratula argentea, with its poor 

 description and execrable figures, should replace T. subtilita, which was well figured 

 and adequatelj' described, it is by no means certain that Shepard's species is the same as 

 that described by Hall, or even congeneric with it. There is little evidence to be derived 

 from the original citation, and none has been produced outside, save the statement 

 by Keyes, that Seminula suhtilita is the commonest species at the locality where T. 

 argentea was first found. This circumstance is of importance, but scarcely, it seems 

 to me, of character sufficiently conclusive to warrant the replacement of Hall's name, 

 which has a definite meaning, for one whose significance is still vague and uncertain. 

 The small size, rotund shape, and elevated beak of the form represented by Shepard's 

 figures seem to indicate, rather than Seminula suhtilita, some other shell, perhaps 

 Heticularia perplexa, to which the two lines of casual description equally apply. 

 Shepard's mention of this shell is formulated in the following words: "It embraces 

 occasionally * * * a small species of Terebratula, whose surface is delicateh^ 

 striated, and of a silvery white color and strong pearly luster." 



Schuchert includes in the synon3'my of this species Terebratula antisiensis and 

 T. peruviana, both of d'Orbigny, which antedate Hall's description bj'' ten years. 

 If he is correct in this, one of these names may have to be substituted for Seminula 

 siibtilita, unless it develops that Shepard's form is the same species. 



It is not improbable that S. caputsenpentis Swallow, 8. singletoni Swallow, 

 8. hawni Swallow, 8. charitanensis Swallow, and 8. diffei^entius McChesney, all of 

 which were described subsequently to 8. subtilita, belong in the synonymy of this 

 species. I also believe that with Seminula subtilita, or, at any rate, with the genus 

 Seminula, belong the shells which Marcou" identifies as Terebratula, planosulcata 

 Phillips and T. royssii I'Eveille, though these have usually been placed with Cleio- 

 thyris, sc. CI. roissyi. 



Locality and horizon. — San Juan region (stations 2186, 2187, 2196, 2196a, 2196b, 

 2196c, 2201, 2202, 2204, 2205, 2209, 2211, 2213, 2214, 2216, 2219, 2220, 2222, 2223, 2224, 



aGeol. North America, Zurich, 185S,'pp. 51, 52, pi. 6, figs. 8, 8a, 8b, and figs. 10, 10a, 10b. 



