MAZATLAN UNIVALVES 409 



species are so numerous and so difficult to identify, tlie grouping 

 of similar forms and tke separation of large assemblages is a 

 gi*eat convenience to the student. Clark, wliose 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. 



Gen CIS OBELISCUS, Humph. 



aray, Proc. Zool. Soc. 1847, p. 159.— S". cj- A. Ad. Gen. vol. i. 

 p. 229. 



Pyramidella, pkrs, auct. — This group is intermediate between 

 the typical PyramideUa; and Odostomia, differing from the 

 former in its smooth surface and scarcely channeled mouth ; 

 from the latter, in its numerous whirls and in the columellar 

 plaits. Several species, very turrited but with a single lamel- 

 liform plait, which are found recent in different seas, and also 

 fossil, may be ranked with either genus. 



486. Obeliscus Pconicus, jun. C. B. Ad. 



Pyramidella conica, C. B. Ad. Pan. Shells, no. 294, pp. 200, 318. 

 Obeliscus conicus, H. Sf A. Ad. Gen. i. 230. 



A solitary yoimg shell, with 5 normal whirls, agrees generally 

 with the Panama species, espcciall}'- 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 lamelliforra. 

 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 and a 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, lat. •055, 

 div. 28». 



Nov. 1856. mm 



