132 EXPLORING EXPEDITION FROM SANTA FE 



be intermediate gradations connecting these extremes with Lamarck's species. At any 

 rate, Mr. Gabb's figures of the septa of B. carinaius and />. asper certainly differ so 



materially from those of our New Mexican specimens (which, as already stated, are 

 quite constant in their internal characters) as to make it desirable that the latter should 

 be designated by another name (at least as a variety), until all doubts in regard to their 

 relations can be removed by a direct comparison of a good series of specimens of all 

 the forms alluded to. 



Locality and position. — Same as preceding. 



Genus PRIONOCYCLUS, Meek. 

 Prionocyclus ? Maoombi, Meek. 



Platell, iitfs. '<\a,b.e,d. 



Shell discoidal ; umbilicus shallow, somewhat less than the diameter of the last 

 whorl from the ventral to the peripheral side, and showing all the inner turns; volu- 

 tions increasing gradually in size, very slightly embracing, compressed so as to be 

 nearly flat on the sides, but rounding into the umbilicus; periphery rather narrow, 

 nearly Hat, and provided with a small mesial carina, which is very slightly waved in 

 outline ; lateral margins of the periphery each having a row of small compressed nodes, 

 arranged one at the termination of each of the COStse, with their longer diameters 

 nearly parallel to the peripheral keel; sides of each turn ornamented by from thirty- 

 six to forty rather obscure, slightly flexuous costae, only every second, third, or fourth 

 one of which extends across to the umbilical margin, where they are usually a little 

 swollen. 



The septa are generally a little crowded in adult shells, and divided into two very 

 unequal principal lobes on each side. Siphonal lobe slightly longer than wide, and 

 ornamented by three branches on each side, the two terminal of which are a little 

 larger and much less spreading than the lateral pair, and each ornamented by some five 

 or six sharp digitations along the margins and at the extremity ; while the first pair of 

 principal lateral branches above the terminal ones are of nearly the same form as the 

 latter, but more spreading, and the third pair are smaller, and merely provided with a 

 few digitations; first lateral sinus (dorsal saddle of old nomenclature) as long as the 

 siphonal lobe, but much wider, and deeply divided into two unequal parts, of which 

 the one on the siphonal side is larger than the other; each of these principal divisions 

 being ornamented by some four or five short, irregular branchlets, with obtusely digi- 

 tate margins; first lateral lobe longer and slightly wider than the siphonal, and pro- 

 vided with some seven or eight short, rather unequal, merely digitate, and palmately- 

 spreading terminal and lateral branchlets; second lateral sinus narrower, but as long 

 as the first on the outer or siphonal side, and much shorter on the umbilical, having 

 two short, unequal, digitate, terminal branches at the end, and some three or four short, 

 irregular divisions along the oblique margin of the umbilical side; second lateral lobe 

 small, or scarcely more than twice as large as the auxiliary lobe of the siphonal sinus, 

 and somewhat irregularly bifid, the divisions being short, and, like the lateral margins, 

 more or less digitate. 



