428 HETEROCENTROTUS MAMMILLATUS. 



the test than in Echinometra. There is do great difference in the structure 

 of the jaws, except that of size in Podophora, Echinometra, and Heterocen- 

 trotus. The tubercles of both areas are of uniform size, forming but two 



vertical rows of primaries. The radioles are very striking and very variable; 

 the large tubercles carry club-shaped or angular massive spines, frequently 

 twice as long as the transverse diameter of the test, apparently smooth, 

 hut in reality finely striated. Round the actinostome the spines are flat- 

 tened, while on the abactinal surface the secondary spines are short, often 

 polygonal, flattened above, and sometimes forming a closed pavement upon 

 the upper part of test, as in Podophora. Auricles tall, slender, with large 

 auricular opening, and connected at the base by a low ridge. 



Heteiocentrotus mamniillatus 



! Cidaris mammillata Ki.kin. J 7:54. Xnt. T > i s | > Ech. 

 Heterocentrotus mammillalus Brandt, [835, Prod. 



PI. TIP.; PI. XXVI. f. l- .'./;; PI. XXXVI. f. g. 



The only two species of this genus I have been able to distinguish have 

 been among the earliest species figured, though the characters upon which 

 they have thus far been separated are not such as can he used with accuracy. 

 The specific distinctions have been almost entirely based upon the differ- 

 ences of the spines, while the only permanent specific characters noticed 

 have generally been adopted as generic distinctions. 



The two species, II. mammillatus and II. trigonarius, have usually been 

 distinguished by their spines: the former species includes all specimens with 

 stout hat-shaped swollen spines, more or less ringed at the extremity; while 

 the last species includes specimens with longer spines, usually tapering rap- 

 idly and more or less triangular. Blainville, Brandt. Agassiz. Valenciennes, 

 and others have attempted to describe other species combining more or less 

 the specific characteristics of the two: specimens having the character- 

 istic test of the II. mammillatus, hut the long triangular or tapering spines 

 of II. trigonarius. have received the name of carinata. hastifera, Postellsii ; 

 while other specimens, having the tuberculatum of II. trigonarius, hut the 

 shorter, bat-shaped, swollen, and ringed spines of II. mammillatus, have been 

 described as violaceus, Blainvillei, etc. The remarkably fine series of 

 specimens of all sizes and localities which I have now had the opportunity 

 of examining leaves but little doubt that there are only two species in 

 this genus, both having nearly the same geographical distribution, and both 



