BRISSUS UNICOLOR. 



Brissus unicolor 



! Brissus unicolor Klein, 1734. Nat. Disp. Ech. 



357 



' PL XXII. f. 1, 2. 



A large number of species of the genus Brissus have been named, but 

 when we attempt to define their specific differences we have but a small 

 number left which can be distinguished with any degree of certainty. A 

 fine series of specimens from the Mediterranean which the Museum owes to 

 the kindness of Professor Panceri and Mr. Rigacci, specimens from the Cape 

 Verde and Canary Islands, the collections of Paris and of Professor 

 Haeckel, show such an extraordinary variation in all the characters which 

 have been employed to distinguish the species of this genus, that I have 

 not found anything left to separate them beyond the usually somewhat more 

 elongate shape of specimens coming from the Mediterranean, while those 

 from the Cape Verde and Canary Islands agreed completely with the 

 West India specimens. The only features by which I am able to sepa- 

 rate the two undoubted species of Brissus (B. carinatus and B. unicolor) 

 are the proportions of the anterior and posterior pairs of ambulacra, and the 

 striking difference in the course of the fasciole in the anterior part of the 

 test. In Brissus carinatus the posterior ambulacra are much shorter than the 

 anterior pair, while they are nearly equal in B. unicolor. There is but one 

 re-entering angle in anterior part of fasciole in the anterior interambulacra, 

 while there are two in B. carinatus ; some minor points of difference in these 

 species will be pointed out hereafter in the descriptions. The outline of the 

 West India specimens is usually elliptical, slightly broader posteriorly ; 

 anal extremity vertically truncated, and apex posterior; test sloping very 

 slightly from abactinal system to apex, forming a broad rounded keel, not 

 specially prominent in the posterior interambulacral space. Posterior extrem- 

 ity of the test uniformly tuberculated, while the larger tubercles of the edge 

 of the lower surface extend over the anterior extremity, slightly beyond the 

 anterior pair of ambulacra, and within the peripetalous fasciole, which is 

 narrow, deeply re-entering in the median lower posterior and odd interam- 

 bulacral spaces. 



Lower surface slightly convex ; actinal plastron elliptical, terminated by a 

 narrow fasciole, forming a well-defined subanal area, elliptical, deeply in- 

 dented below the anal system, broader than the actinal plastron. Anal 

 system large, elliptical, composed of one exterior row of large polygonal 

 plates; interior ones smaller and irregularly arranged. 



