MAZATLAN UNIVALVES 
On 
bo 
me 
Suscenus PHYLLONOTUS, Swans. 
Varices numerous, often irregular; foliated or branching. 
666. PHyLLONOTUS NiGRITUS, Weusch. 
Murex nigritus, Phil. Abbdild. pt. viii. p. 3, pl. 1, f. 1.—Rve. 
Conch. Ic. pl. 42, f. 47.—Mke. in Zeit. f. Mal. 1850, p. 188, 
no. 73. 
+ Murex ambiguus, Rve. Proc. Zool. Soc. 1845, p. 86 :—Coneh. 
Lc. pl. 18, f. 51.—Mke. in Zeit. f. Mal. 1850, p. 188, no. 74:— 
(?non C. B. Ad. Pan. Shells, p. 122, no. 138, var.) 
_ Comp. Murex radix, Gimel. 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, 
