198 PEOF. p. MARTIN DUKCAN ON THE 



and some of these are to tlie median line of, or in the inner and 

 peristomial part of, the base of a process (fig. 26). Usually 

 there are the relics of four grooves with their pores (some often 

 absent) on one side of the peristomial edge and on the base of a 

 process, and of three grooves and their pores, more or less com- 

 plete, in the base of the other process of the arch ; that is, ambu- 

 lacral plates 1-4 and 1-3 in the respective zones. On looking at 

 a process from behind, much crowding of plates and pairs of 

 pores is seen, and at least three pairs of pores, representing as 

 many plates, are placed obliquely at the base and at the inner 

 flank of the process ; they (fig. 30) correspond in their position 

 to the pairs of Echinus. Hence in Psmnmechinus the base and 

 much of a process is made up of three plates visible from behind, 

 which are the plates 4, 5, and 6, or in the opposite process 

 plates 5, 6, and 7. Besides these, there are the plates seen in 

 front or at the peristomial edge, numbering 1-3 or 1-4. A 

 process in Fsammechinus may therefore be composed of the 

 oblique and hypertrophied poriferous parts of seven plates. 



There is the same disposition to fracture in the processes above 

 the spot of the uppermost externally visible pores ; but no sutures 

 can be seen with even the aid of benzene. That there are canals 

 passing out of sight in processes and communicating with pores 

 is evident ; for on fracturing a process moderately high up a pair 

 of canal-ends became visible (fig. 31). (This may be also seen in 

 Strongylocen trottis.) 



The ridges are moderately high and are broadest inferiorly ; 

 the upper edge is concave, and there may be swellings on it close 

 to the median line. The branchial grooves and other features of 

 the peristomial face of the ridge are as in Echinus. The con- 

 struction of a ridge is very much the same as in Echinus (figs. 27, 

 28, 29). 



V. The Echinometbad^ and the Diadematid^. 



Strongylocentrotus lividus, Lmk., sp. — The perignathic girdle 

 of this species is slender ; the processes are moderately tall, 

 slender, united above, without much lateral expansion there, and 

 they have narrow and tall openings. There is much crowding 

 of the ambulacral plates at the base of a process ; and three 

 or four pairs of pores may be seen from behind to be on the 

 base, back, and inner flank of the process. There are as many 

 plates in front. The ambulacro-interz'adial sutures of all these 



