BRISSUS OBESUS. 597 



adult, where it is usually narrow and sharply defined. The spines of this 

 species are perhaps slightly stouter than in the other species ; they vary in 

 dried specimens in color from a dark brown to a uniform silver-gray. The 

 outline of the test from above varies from greatly elongate, pointed poste- 

 riorly, with the greatest breadth across the apical system, to a nearly 

 pentagonal outline with rounded anterior edges, and the greatest breadth 

 across the extremity of the posterior lateral ambulacra. This species reaches 

 a large size : specimens measuring 140 mm in length are in the Museum col- 

 lection. The subanal plastron is less in width than the broadest part of the 

 actinal plastron ; the actinal surface increases in convexity with age. 



Long. Diam. Trans. Diam. Height. 



140. 110. 80. 



123. 98. 74. 



110. 97. 61.5 



92. 73. 47.8 



69. 49. 40. 



53. 40.5 28. 



Society Islands ; Sandwich Islands ; East India ; Mauritius ; Philippine Islands. 



Brissus obesus 



I Brissus obesus Verrill, 18C7, Notes on Radiata, p. 316. 



This species has been described by Professor Verrill, and distinguished 

 from B. unicolor by the more evenly rounded outline between the anterior 

 margin and ovarian plates, the more swollen posterior region, the increased 

 convexity of the subanal and ventral areas, the less anterior position of the 

 abactinal area, and the relatively shorter and broader subanal plastron. 

 The difference in size of the genital openings and of the abactinal sys- 

 tem in specimens of the same size, the slightly narrower lateral ambu- 

 lacral petals ami larger anal system in B. obesus. seem the most constant 

 characters. It may perhaps turn out that this species is only a variety of 

 Brissus compressus, which I have united with B. carinatus. As I have al- 

 ready stated, when describing B. unicolor, I find great difficulty in distin- 

 guishing the species of Brissus, and I have been led to unite as one species 

 the Brissidae described from the Mediterranean and West India Islands. 

 With ampler material at our command the question of the identity or differ- 

 ence of the species found on the two sides of the Isthmus of Panama might 

 be decided. This is desirable, as it is one of the few species of Echini where 

 the discrimination of the Atlantic and Pacific representatives is somewhat 

 doubtful. 



Panama : Gulf of California. 



