MAZATLAN UNIVALVES 409 
species are so numerous and so difficult to identify, the grouping 
of similar forms and the separation of large assemblages is a 
great convenience to the student. Clark, whose researches 
among the animals of this tribe are most minute and valuable, 
regards the presence or absence of a columellar plait as a 
variable character even in the same species: v. Moll. Test. 
Mar. Brit. pp. 395—454. The Mazatlan species appear con- 
stant in this character; although the plait, when present, is 
often hidden, and cannot be seen except in broken shells, 
which thus assume a more than usual value. One of the best 
characters is found to be the shape and size of the sinistral 
portion: the first observations of it however must be taken 
with caution, as slight changes in the light or angle of observa- 
tion cause very different appearances under the microscope. 
, * 
Genus OBELISCUS, Humph. 
Gray, Proc. Zool. Soc. 1847, p. 159.—H. & A. Ad. Gen. vol. i. 
p. 229. 
Pyramidella, pars, awct.—This group is intermediate between 
the typical Pyramidelle and Odostomia, differmg from the 
former in its smooth surface and searcely channeled mouth ; 
from the latter, in its numerous whirls and in the columellar 
plaits. Several species, very turrited but with a single lamel- 
hform plait, which are found recent in different seas, and also 
fossil, may be ranked with either genus. 
486. OBELIScUsS Pconicus, jun. C. B. Ad. 
Pyramidella conica, C. B~Ad. Pan. Shells, no. 294, pp. 200, 318. 
Obeliscus conicus, H. & A. Ad. Gen. i. 230. 
A solitary young shell, with 5 normal whirls, agrees generally 
with the Panama species, especially in having the suture in 
a groove which travels round the periphery, and in having two 
very small plaits under one which is large and lamelliform. 
It differs in the base which is scarcely angulated, and in the 
divergence which is greater. Both these features may alter in 
the adult shell. The apex might appear “very acute” in an 
adult of 14 whirls, though it is rather obtuse in this specimen ; 
the nuclear whirls (one anda half) which are like a tumid 
Planorbis, being sunken in the normal portion, of which it 
covers about two-thirds. Long. ‘105, long. spir. ‘065, Jat. ‘055, 
div. 28°. 
Nov, 1856. mm 
