PALEOSTOMA MIRABILIS. 583 



ever, can only be hoinologized to the regular large buccal tentacular plates 

 which are found in all young Echini, and which, in many genera, always 

 retain a greater preponderance, and in the young cover nearly the whole of 

 the actinal membrane. Though in young Spatangoids Ave find a smaller 

 number of actinal plates, yet nothing thus far has been observed in which 

 a regular radial arrangement exists ; all the Spatangoids with pentagonal 

 actinostome having a membrane covered with a large number of small plates 

 of nearly uniform size. 



Paleostoma mirabilis 



! Leskia mirabilis (Jrav. 1851, Ann. Mag. N. H. 



! Paleostoma mirabilis Lov^N, 1867, Vet. Ak. Forhdl., p. 432. 



PI. XXXII. f. 13-15. 



The specimens of this species in the British Museum are smaller than the 

 figures given by Gray, in his Catalogue of Recent Echini, and show marked 

 differences from the young specimens collected by Kinberg, in the Stock- 

 holm Academy Museum, from which the Cambridge Museum owes a couple 

 of specimens to the kindness of Loven. 



In the specimens described by Gray, which are by no means adult (as 

 shown by fragments collected by Simpson), the test is ovoid, with a rather 

 angular outline ; the vertex is nearly central, slightly anterior. The lateral 

 ambulacra arc broad, petaloid, rounded at the extremity, somewhat sunken. 

 The posterior pair much the shortest ; the odd anterior ambulacrum placed 

 in a rather broad, very slightly sunken, groove, with rudimentary porif- 

 erous zones of vertically distant pores ; peripetalous fasciole broad, some- 

 what angular, following in its general curve the outline of the test. There 

 are two very large, transversely elliptical, ovarian openings; actinal surface 

 convex ; actinal plastron oval, but not differently tuberculated from the rest 

 of the test, hence does not appear in striking contrast to the rest of the 

 actinal surface, as in other Spatangoids. 



Anal system circular, placed near the upper extremity of the rounded 

 posterior extremity, covered by six to seven large triangular converg- 

 ing plates ; the anal opening itself surrounded by a number of minute 

 pointed plates. The actinostome is anterior, pentagonal, flush with the sur- 

 face of the surrounding test ; the angles of the pentagon corresponding with 

 the ambulacra. The sides of the pentagon form the base of short triangular 



