MUSEUM OF COMPARATIVE ZOOLOGY. 255 



at once gives us the key to the position of an anterior and posterior side 

 among Sea-urchins. The correctness of this view is fully maintained from 

 the analysis of the abactinal system of a living Salenia here described, which 

 shows that the sub-anal plate is the homologue of the first anal plate of 

 young Echini, (which in many cases remains decidedly larger in older stages, 

 — Toreumatica, Genocidaris, Trigonocidaris,) and shows that the abac- 

 tinal system of Salenia is entirely homologous with the abactinal system of 

 the Echinoids, the original plate only retaining a greater preponderance 

 than has thus far been noticed in other genera. The remaining part of the 

 anal system was, in the fossil species, undoubtedly covered by small plates, 

 as in the living species ; and that this was the structure of the anal system 

 is shown by Wright, who has figured the abactinal system of Acrocidaris, 

 and removed the genus to Salenida? on account of the presence of a sub- 

 anal plate. This feature, which seemed so characteristic of a small group of 

 Echini, is one which alone has no systematic value, so that we must, I think, 

 hereafter consider the Salenidae simply as a sub-family of Cidaridas, as 

 the description of the species dredged in Florida by Mr. Pourtales will 

 clearly show. 



The general appearance of Salenocidaris is that of a young Dorocidaris 

 •abyssicola. The primary spines are enormous, — twice the diameter of the 

 test in length, of a brilliant white color, and of all shapes. Some of them 

 are uniformly tapering, others swelling at about one third the distance 

 from the base, others flattened and curved, but all finely longitudinally 

 serrated with sharp spines, irregularly arranged along the body of the 

 spines. The secondary spines, as well as the greater number of the spines 

 of the ambulacra, as far as the ambitus, are short, club-shaped, sometimes 

 curved and flattened, longitudinally striated with slight serrations. These 

 short spines give to the median interambulacral and ambulacral zone the 

 aspect of the corresponding zones of Cidaris ; but they are not, as in Cidaris, 

 arranged in a circle round the base of the primary spines. These small 

 spines, as well as the whole abactinal area, are covered with prominent 

 dark violet pigment cells, standing in striking contrast to the white pri- 

 mary spines. The abactinal system has the structure of that of Salenia, 

 but the position of the anal system is that of Hyposalenia. As we know 

 nothing of the spines of either of these genera, it is better for the present to 

 establish a new genus founded upon this peculiarity of the abactinal system, 

 and the imbricated buccal membrane, which is covered thickly with plates 

 arranged somewhat as they are in Echinocidaris ; the ten buccal plates are 

 sparingly covered by pedicellarias. The primary tubercles of the interam- 

 bulacral area are large, arranged in two vertical rows in the two areas ; 

 those of the ambulacral area are smaller, and diminish rapidly towards the 

 abactinal pole ; the median interambulacral space is occupied by two ver- 



