ASTHENOSOMA HYSTRIX. 3 



Asthenosoma hystrix 



I Calveria hystrix W. TiiOMS., 1SC9, DiiMlg. Kcp. PorcuiJine. 

 \ Ast/ienoKoma lii/.ilri.c A. Ac, 1872, Rev. Ech., I't. I. p. 93. 



PL 11. f. 1,2. 



A fine specimen of this species was dredged ofl" the Barbadoes in 100 

 fathoms. This has enabled me to examine more in dettiil this remarl<;able 

 sea-urchin, and to satisfy myself of the correctness of the systematic views 

 held by Thomson regarding it. In the Revision of the Echini I still included 

 the genus among the Diadematidae. I am now convinced, from study 

 with better material, that Thomson is correct in separating these Echini as a 

 distinct family from the Diadematidae, for which he proposes the name of 

 Echinothuridae ; in his Preliminary Report* he has given some of the rea- 

 sons for this course ; they are to be found more in detail in the Depths of 

 the Sea. f The association with Echinothuria may )je somewhat doidjtful, 

 as we hardly know the genus accurately enough for such an intimate asso- 

 ciation, in spite of the ingenious and careful examination made of the frag- 

 ments by Woodward, t Echinothuria has, in common with Asthenosoma, 

 the reverse lapping in the coronal plates and in the buccal membrane ; but 

 the structure of the ambulacral and interambulacral systems can only be 

 guessed at in the fragments of the fossils which have thus far come to light. 



The separation of this family from the Diadematidae is made on account 

 of the mailed structure of the coronal plates lapping in opposite directions 

 in the ambulacra and interambidacra, on accomit of the perforated andau- 

 lacral plates, and the extension of the ambulacral tubes to the actinal open- 

 ing, through the buccal membrane, which is mailed as in Cidaris. 



The specimen figured in Pi. II. f. i, 2 was, when brought up in the 

 dredge, of a deep claret color. The test is perfectly flexible, owing to the 

 lapping of the plates and the deposition of the limestone only in certiiin 

 parts of the ambulacral and interambulacral plates, leaving a part of the 

 edges of the plates, where they do not lap, free from limestone. Seen from 

 above, the outline is pentagonal, with rounded corners ; the central part of 

 the test is depressed from its own weight. The contrast between the actinal 

 and abactinal surfaces is very marked, owing to the flatness of the test, — the 

 height of the edge of the test not equalling more than one fifth its diameter. 

 The width of the ambulacral zone at the ambitus is about half that of the 

 interambulacral area. The number of plates along the median ambulacral 



* W. Thomson, 1873, Preliminary Report of the Porcvipinc Ecliinodenus. Proc. R. S. 

 t '■ " Depths of the Sea. Figs. 27, 28, pp. 15G, 157. 



X WooDWAiiD, Geologist, September, 18G3. 



