A. Agassiz — JEchinoneus Van JPhels. 491 



Fig. 10 is a profile view of a young specimen 5*16 mm in 

 length, in which the jaws, lantern and pyramid have disap- 

 peared ; the lettering of the parts is as in fig. 1. 



Fig. 3 shows the lantern more in detail ; it is l*3 mm in diam- 

 eter ; oes is the opening of the oesophagus leading into the 

 lantern ; jpm, the horizontal muscular band of the lantern ; 

 c, the compass ; jpy, the pyramid ; a, a, a, a, the small auricles ; 

 mc, the muscular band of the compass ; I, the labium. 



Fig. 4 shows the position and shape of the auricles, «, of a 

 specimen 4-10 mm in length. 



Fig. 5 shows the pyramid with the tooth (t) seen from the 

 grooved side, while fig. 6 is the same showing the tooth from 

 the keeled side. The pyramids are solid without foramina. The 

 shape of the compass is shown in figs. 8 and 9, the profile and 

 dorsal views of the same. 



The appearance of the young Echiuoneus after the disap- 

 pearance of the lantern is shown in a profile view of the same, 

 fig. 10. How this disappearance takes place I am unable to 

 state, the material at my command showing no intermediate 

 stage between that of fi.g. 1 and of fig. 10. It is possible that 

 the lantern disappears by resorption ; the whole of the calca- 

 reous deposits in the masticatory apparatus is most loose and 

 delicate. Its resorption would be in harmony with the resorp- 

 tion of many parts in the test and spines of other Echini. 



It is interesting to note that this dentate stage of Echino- 

 neus should connect the embryonic Spatangoids with the early 

 stages of Clypeastroids and of the Echinoconidse, while the 

 toothed Pygastroides of Loven connects it with some of the 

 Echinoconidge ; both Echiuoneus and Pygastroides are West 

 Indian genera. 



The presence or absence of auricles, teeth and pyramids 

 forms the basis of Zittel's classification of the irregular Echini 

 into Gnathostomata and Atelostomata. The position of the 

 anal system on the actinal surface close to the actinostome is a 

 very modern feature : its gradual passage from an anal system 

 enclosed within the calycinal system as in Pygaster, where it is 

 outside of the abactinal system, can be traced towards the 

 ambitus to the ambitus and finally to an actinal position. 



In some of the Holectypidse the jaws are greatly reduced in 

 size and solidity and their importance much lessened, and in 

 others the jaws have entirely disappeared, the perignathic 

 girdle having become rudimentary. In the Echinometradse 

 the auricular girdle is most highly developed. In the Holec- 

 typidse we find primitive irregular Echini as well as types 

 which have persisted to the present day. In the Discoidea and 

 Galerites the ambulacral processes and teeth are absent ; in the 

 auricular girdle the processes become low. In the Clypeas- 

 troids there are neither braces nor compass, the jaws move 



