MAZATLAN UNIVALVES 521 



PHYLLONOTUS, Swain*. 

 Varices numerous, often irregular ; foliated or branching. 



666. PHTLLONOTUS NIGBITUS, Meuscli. 



Murex nigritus, Phil. Ablild. pt. viii. p. 3, pi. 1, f. 1. Rve. 



Conch. Ic. pi. 42, f. 4ff.Mke. in Zeit. f. Mai. 1850, p. 188, 



no. 73. 

 + Murex ambiguus, Hve. Proc. Zool. Soc. 1845, p. 86: Conch. 



Ic. pi. 13, f. 51. Wee. in Zeit.f. Mai. 1850, p. 188, no. 74 : 



(? non C. B. Ad. Pan. Shells, p. 122, no. 138, var.) 

 Comp. Murex radix, Gmel. p. 3527, no. 10. Lam. An. s. Vert. 



vol. ix. p. 584, no. 36. C. B. Ad. Pan. Shells, loc. cit. 



The form ambiguus is distinguished from the M. nigritus of 

 Phil, by the broader shape, eight varices, more pinnate expan- 

 sions, and deeper colour in the mouth. Dr. Menke prides 

 himself on his superior facility for discriminating the species, in 

 consequence of possessing no fewer than 8 very fine specimens. 

 Having carefully examined many hundred shells in the finest 

 state of preservation, I am unable to substantiate the difference. 

 The great bulk of the adult Mazatlan specimens belong to the 

 form ambiguus, which is the typical state of the species, when 

 grown under the most favourable circumstances. The form 

 nigritus, as figured by Philippi and Reeve, of which very few 

 adult specimens occurred in this collection, appears to be 

 caused by a deficiency in what the animal requires for its full 

 development. Almost all the young specimens however are 

 of the form nigritus, only one having been found with finely 

 pinnate varices. The number of spiny processes varies greatly 

 in different shells, as also does the amount of their pinnation. 

 The operculum, which is remarkable, presents exactly the 

 same characters in the young and adult nigritus, and in the 

 ordinary ambiguus. The amount of elevation in the spire is 

 extremely variable in the form ambiguus ; it is generally much 

 depressed, as in M. radix, but sometimes pointed, and with the 

 outlines incurved. The number of varices varies from 6-9 in 

 the young shells ; from 7-9 in the adult nigritus ; and from 

 8-13 in the adult ambiguus. Whether the species be distinct 

 from M. radix, cannot yet be determined. Individuals of each 

 closely approach each other. Thus far however the provinces 

 are distinct, M. radix not having been found north of Acapulco, 

 nor M. nigritus so far south. The form radix is much heavier, 



