490 H. "Sr. SHIMEE PEEMO-TEIASSIC OF XOETH^VESTEEX AEIZOXA 



millimeters ) in height of shell and width of apeiture 15 by 15 — : the 

 largest specimen in onr collection is 20 by 20-|-. Umbilicus distinct, 

 though small. The peritreme expands rapidly and is broadly rounded 

 across the dorsnm. Slit band is sharply defined, but not much elevated. 

 Inner lip with a thick deposit of callus. 



In sculpture our specimens, like those noted by Girty from the ^Ianzan<;» 

 group of Xew ]kIexico/^ have six revolving striae in the space of 5 milli- 

 meters, but they have one to six less regularly arranged transverse stria? 

 in 5 millimeters. The transverse stride are bent backward as they approach 

 the slit band. Xear the aperture, the first half inch in mature specimens, 

 the transverse stride are more regularly arranged and more numerous, 

 5 to 6 in 5 millimeters. Upon the remainder of the coil, back to tlie 

 callus, these become broad, rounded, irregular undulations, 1 to 2 in 5 

 millimeters, though at times a single undulation will be made up of two 

 or three transverse stris. In addition to these stride and imdulations. 

 growth lines are prominent. In young shells, under 10 millimeters in 

 diameter, these growth lines are the only transverse ornamentation; 

 beyond this size the coarse, transverse undulations make their appearance. 

 The regular, conspicuous revolving striae occur on the youngest shell 

 noticed. As in the type of this species, the revolving stride become 

 strongest upon the crests of the transverse striae, forming there slight 

 nodes. 



Possibly some of our small specimens may belong to Bucanopsis modesta 

 Girty/- but since there are in our collections all stages, from sheUs with 

 no transverse striae, or undulations, to those exhibiting these in typical 

 development, it is thought best to consider all as the young of P. nodo- 

 costatum. 



Our form differs from P. montfortianum X. e\: P. in that upon the 

 latter the revolving striae are of two sizes, the large ones alternating with 

 several smaller ones. In P. ourayethse Gurlev a cross-section of the 

 peritreme is deltoid, pointed at the slit band. In P. helium Keyes the 

 transverse markings go nearly straight across the shell and are not curved 

 backward at the slit band. As there is no published figure of P. nodo- 

 costatum. definite identification is difficult and our form may prove to be 

 a distinct species. 



Cf. Patellostium ourayense (Gurlev). — A cross-section of the peritreme 

 of Gurlers type specimen^^ shows the sides compressed and narrowly 

 rounded at the slit band — that is, the section is sub-deltoid. A few of 



^T. S. Geol. Surrey, BaU. 3S0. 1909. p. 102. 



" Op. cit.. p. 10.3. pi. 11. fig. 1. 



13 Girty : F. S. Geol. Surrey. Prof. Paper 16. 1903. p. 471. pi. 10. fig. 10. 



