254 PROCEEDINGS OF THE 



by Professor Sir Wyville Thomson, in the " Depths of the Sea," 

 under the names of Calveria hystrix and Phormosoma i)lacenta. 

 The specimen from Dr Thomson's collection was probably 

 obtained at an earlier date than the Porcupine's cruise, and 

 appears to belong to a different genus from the above. Its 

 locality is not known, and from its appearance it seems to have 

 been lying past for many years, having lost its spines and 

 dental apparatus, though otherwise the test is quite perfect. In 

 its depressed form and flexible test it presents a strong contrast 

 to the rigid shelled species of Echinus found on our coast, but in 

 other respects clearly belonging to the same family group of the 

 Echinodermata. The specimen measured 18| inches in circum- 

 ference by 2J inches in height. The test' is thin and flexible in 

 all its parts, but its upper surface is much more elastic than the 

 lower. As in other members of the same group, the test is 

 composed of five double rows of ambulacral and interambulacral 

 plates, all of which bear perforated tubercules, to which the 

 spines were attached. The plates, so far as yet examined, appear 

 to be arranged in an over-lajiping series, as in Calveria and 

 PJiormosoma, but it is in the character of its interambulacral plates 

 on the upper surface of the test that Dr Thomson's specimen is 

 seen to diff'er from the above genera. These plates are divided 

 into three areas, the middle space being flat, and free of tubercules. 

 This peculiar character commences at the periphery of the test, 

 where the middle area of the plate is seen to widen in each 

 successive row, by the reduction or loss of the side tubercules 

 towards the summit, where the genital disc is placed. At this 

 point the primary tubercules disappear, leaving a wide flat space 

 in each of the ambulacral rows, which, being of a sj^ongy, 

 membraneous texture, gives great flexibility to those plates. The 

 interambulacral plates on the lower surface of the test have no 

 intermediate flat spaces, the tubercules being continuous across. 

 The ambulacral plates are perforated each by three pairs of pores, 

 two pairs on the lower edge of the plates, and one pair on the 

 upper edge, intermediate in position between the other two ; tliey 

 are likewise continuous in their rows, from the mouth to the 

 summit of the test. Mr Young stated that he had already 

 pointed out in the Transactions of the Society that Archaeocidaris 

 Urii, one of the oldest of our fossil sea urchins, found in the 

 limestone strata of the Glasgow district, had also a flexible test 



