ECHINOIDEA 321 



Key to the families of Amphisternata known from or likely 

 to exist in the British seas. 



1. Peristome circular,^ first plate of posterior interambulacrum not 



forming a labrum .... Fam. Aeropsidce 



Peristome transversely elongate ; first plate of posterior inter- 

 ambulacrum usually forms a labrum .... 2 



2. Ambulacra not petaloid . . . Fam. Palceopneustidce 



„ petaloid ....... 3 



3. Subanal fasciole wanting ... I. F^m. Hemiasteridce 



,, ,, present . . . II. Fam. Spatangidce 



[Fam. Aeropsid^] 



This family comprises some peculiar small deep-sea forms 

 characterised mainly by the primitive structure of the peri- 

 stome, which is circular, covered by a membrane containing 

 some few scattered plates or none. The first plate of the posterior 

 interambulacrum is not modified so as to form a prominent 

 labrum. A peripetalous fasciole is present. The tube-feet of the 

 frontal ambulacrum very large, with an expanded disk. 



The two genera known both occur in the Atlantic, viz. 

 Aeropsis Mrtsn. (Syn. Aerope Wyv. Thomson), with the species 

 A. rostrata (Wj^r. Thomson),^ and Aceste Wyv. Thomson, with 

 the species A. bellidifera Wyv. Thomson. Aeropsis rostrata has 

 been found in the Davis Strait (" Ingolf "), and off the east coast 

 of N. America (' Challenger "), in depths of ca. 2230-3150 m. 

 It has also been recorded from the Bay of Biscay and off the 

 Portugal coast, but these localities are scarcely reliable. Aceste 

 bellidifera was found off the Canaries, 1116 m. (" Challenger "), 

 and off the Azores, 1360 m. (" Princesse Alice "). 



The two genera are thus distinguished ^ : 

 Test elongate, more or less cylindrical ; peristome flush with ventral 

 surface ; frontal ambulacrum not deepened Aeropsis Mrtsn. 



Test wide and flattened ; peristome vertical ; frontal ambulacrum 



greatly enlarged and depressed . . Aceste Wyv. Thoms. 



^ The circular peristome is an embryonic character, found also in the 

 newly metamorphosed young of other Spatangoids, but in these latter it 

 very soon (in specimens of 2-3 mm. length) begins to assume the character- 

 istic transversely elongate shape, whereas in the Aeropsidse it retains the 

 round shape throughout life. 



^ H. L. Clark gives A. M. Norman as author of the name Aerope 

 rostrata, but as Norman himself gives Wyv. Thomson as author of the name, 

 the fact that Norman's description antedates Wyv. Thomson's by some 

 months woiild not seem to justify depriving Wyv. Thomson of this honour. 



^ For detailed descriptions and figures of the two species see 

 "Ingolf' Echinoidea, ii., p. 90, Pis. V., XV. ; " Challenger " Ech., pp. 192, 

 195, Pis. XXXII., XXXIII. 



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