44 ECHINOIDEA. II. 



specimens the fascicle is generally distinct, whereas in larger specimens it gradually becomes less 

 distinct on account of numerous small miliary spines, like those of the fasciole, developing between 

 the primary spines on the adjacent part of the plastron. Loven is scarcely right in maintaining that 

 the extension of plates I. a. 4 and V. b. 4 can in no wise be compared with the extension of plates I. a 

 6 + x and V. b. 6. -|- x - m Prymnodesmic Spatangidse. It is a fact of importance for this question that 

 in Brissopsis (Toxobrissus) pacifica and the species elongata described below the first extended plate 

 is not the 6th but the yth a case unknown to Loven 1 ; had he known this, he would probably not 

 have laid so much stress on the numero 6. I think it not unreasonable to conclude that, when the 

 subanal fasciole of the Prymnodesmic Spatangidse includes sometimes the plates I. a. 7 -j- x and V. b. 7 

 + x instead of 6 -j- x (and nobody will doubt the homology of the fasciole and extension of plates in 

 this case), it may also be possible to regard a fasciole including only the extension of plates I. a. 4 and 

 V. b. 4 as homologous with that of the Prymnodesmic Spatangidse 2 and that likewise will hold 

 good for the extension of this plate, in case the fasciole is wanting, whether it has disappeared with 

 age or was never formed. That only one plate extends so as to reach within the fasciole cannot be 

 against the homology. In the young Echinocardium cordatum likewise only one plate extends within 

 the fasciole, viz. the 6th, as is described below. The fact that only one plate extends within the fasciole 

 in U. naresianus thus evidently marks the fasciole of this species as being very primitive and of 

 an embryonal character. Otherwise, if it be right what Lambert (Etudes sur le plastron des 

 Spatangides) and de Meijere (Siboga-Ech. p. 153) maintain that the Amphisterni have not devel- 

 oped from the Meridosterni, (and I, for my part, am fully convinced that they are right herein), the 

 fasciole evidently will have developed independently in each group, and it is thus not surprising to 

 find some differences in its relations in the two groups. Be that as it will, it is certain that the forms 

 without a subanal fasciole agree exactly with those provided with a fasciole in the structure of the 

 ambulacra of the bivium; there cannot be distinguished two groups, one without, the other with a 

 subanal fasciole, as was suggested by Duncan. 



Nevertheless Duncan was certainly right in suggesting that Agassiz has confounded two 

 species under his Ur echinus naresianus in the ^Challenger -Report On an examination of the speci- 

 mens of Urech. naresianus in the British Museum I find that those from St. 158 are not really that 

 species; their globiferous pedicellarise differ so considerably from those of naresianus, that they can 

 certainly not belong to this species; they agree exactly with those of Cystech. Wywillii (comp. below 

 p. 49). Probably these specimens will prove to belong to this latter species; since, however, Cyst.Lovcni 

 and Urech. giganteus also have similar globiferous pedicellarise, I shall not try to decide to which 

 species these specimens really belong, but be satisfied with having shown that they are not naresianus. 



As pointed out by Loven it is the 4th ambulacral plate in the series La and V. b which ex- 

 pands internally to meet the episternal angle, and this is a very constant feature. Among the nume- 

 rous specimens I have examined, I have found only two exceptions: in one case the plate I. a. i is 

 abnormally divided into two plates with one tentacle each, the plate with the episternal prolongation 



1 Also in Micraster corangumum there is some irregularity in this respect, it being the V. b. 5 which reaches the 

 fasciole according to the analysis of the test given in Lo yen's: Etudes. PI. XXXIII. 



2 In Ureckinus giganleus it is the 6th plate which is extended (Panamic Deep-Sea Echini, p. 154. Fig. 221); no fasciole, 

 however, has been observed in this species. 



