522 MAZATLAN UNIVALVES 



with slightly developed but very numerous varices. The shells 

 doubtfully assigned to Mr. Reeve's ambiguous species by Prof. 

 Adams, appear to be finely grown specimens of M, radix.* 



Shell of light growth, with a variable number of varices, 

 generally 7 in the young and 9 ^n the adult state, with a vari- 

 able pattern of open, more or less pinnate fronds, black within. 

 These fronds are the expansions of rounded spiral ribs. The 

 whole surface often very finely spirally striated. Base of 

 columella twisted : canal nearly covered, bent, leaving a rather 

 large spiral umbilicus. Labium generally stained with black. 

 Groundwork of the shell white, with black ribs. Operculum 

 ovate, much bent, nucleus near the margin, with irregularly 

 laminated ridges of growth ; on the outer border rising into 

 very crowded erect serrated laminae ; colour dark horny brown, 

 scarcely tinged with orange or red ; muscular scar large, 

 corrugated like the human hand from a center on the inner 

 margin. The smallest of the specimens, (including spines,) 

 measures long. 1*75, long. spir. '52, lat. 1'25, div. 80. 



The largest 7', 1'7 57, ,,100. 



Hob. Mazatlan ; abundant and extremely fine, among very 



fine sand ; L'pool fy Havre Coll. 



Young state. Tablet 2481 contains 5 young sp. passing from 

 form nigritus to form ambiguus. 2482, 1 sp. jun. six varices. 

 2483, 1 do. six and a half. 2484, 1 do. seven. 2485, 1 do. seven 

 and a half. 2486, 1 do. eight. 2487, 1 do. eight and a half. 

 2488, 1 do. nine. 



Form nigritus, Phil. 2489, 1 sp. seven varices. 2490, 1 do. 

 very fine, seven and a half. 2491, 1 do. eight. 2492, 1 do. 

 from a shop, locality unknown, displaying nine varices. 



Form ambiguus, Eve. 2493, 1 sp. dark, smooth, (lip rather 

 broken) eight varices. The middle part of the last three 

 varices takes the form nigritus, in consequence of an accident. 

 2494, 1 sp. nine varices. 2495, 1 sp. labium scarcely stained, 

 spire outlines excurved, ten varices. 2496, 1 sp. (lip rather 

 broken) labrum dark, eating away the lower part of an old 

 varix, eleven varices. 2497, the largest sp., twelve varices. 

 2498, 1 sp. thirteen varices. 



Almost all the above specimens have opercula. Tablet 2499 

 contains 3 others, of which two are distorted, with subcentral 

 nucleus. 



* Of this species, though now extremely common, Lamarck thus writes : 

 <c Shell very rare and very precious. I do not possess it j but I have had an 

 opportunity of observing and examining its characters." 



