ASTHENOSOMA HYSTRIX. 273 



Asthenosoma hystrix 



! Calveria hystrix W. Thomson, 1869. Prelim. Rept. Porcup. Exped Proc. R. S. 

 1 Asthenosoma hystrix A. Ai>. 1872. Rev. Eeh. Pt. I. 



PL IP. f. 1-5. 



The probability of the correctness of the view which I took in the Pre- 

 liminary Report, of referring a small Sea-urchin to Calveria hystrix, is greatly 

 strengthened by finding among some refuse of the dredgings from Florida, 

 collected by Mr. Pourtales, a couple of plates of Asthenosoma hystrix. They 

 must have belonged to a specimen measuring about one and a half inches in 

 diameter, to judge by a specimen slightly larger, which the Museum owes to 

 the kindness of Professor Thomson, collected by the Porcupine Expedition. 

 The small spines on this specimen show the same structure as those of the 

 young Sea-urchin referred to (Asthenosoma), and figured on PI. IP. f. 3. 

 Older spines become somewhat trumpet-shaped (PI. IP./. 4). The peculiar 

 structure of these spines is well shown in the transverse section (PL IP.f.5), 

 — a structure identical with that of the spines of Asthenosoma varium, which 

 I owe to the kindness of Professor Grube of Breslau. The pedicellariae also are 

 similar, leaving but little doubt of the generic identity of these two remark- 

 able Sea-urchins. 



The young specimen, briefly mentioned in the Preliminary Report, is a 

 small Sea-urchin, about 3 mm - in diameter (PL IP. f. 1, 2). It is already 

 of a size when a young Diadema has its plates tolerably well defined, and 

 when its spines far surpass the diameter of the test in length, besides 

 being provided with a long anal proboscis, which at once characterizes young 

 Diadematidae This specimen was nearly flat, the outline deeply cut at the 

 ambulacra (PL IP./, l), the interambulacra projecting as large lobes beyond 

 the general outline ; the whole test was made up of small limestone cells 

 (PL IP. f. 2), and evidently was quite movable, though tough in its present 

 condition ; there were deep actinal cuts in the centre of the ambulacral field 

 (PL IP. f. l), the actinal membrane unfortunately was not well preserved. 

 The spines were very short (PL IP./. 3), and bore about the same ratio to the 

 test which they have in Astropyga and Asthenosoma, the tubercles were not 

 yet separated from the general limestone network of the test, and the spines 

 were arranged in the interambulacral spaces in two irregular main rows, and 

 in one row in the ambulacral spaces, both extending to the abactinal pole 

 from the ambitus, the test round the actinostone being bare (PL IP./, l). 



Florida Gulf Stream, 138 fathoms. 



