208 PROF. p. MARTIN DUNCAN ON THE 



In the TemnopleuridsB the retractor muscles are attached to 

 " processes," one on each side of an ambulacrum ; aud they join 

 above in an arched form, and the ambulacrum forms the floor of 

 the arched space. 



The processes are growths of the poriferous portioois of the 

 amlulacral plates near the peristome ; and the base of a " process " 

 is united by suture with the "ridge" oii the interradium, the 

 line of direction of the suture being along the ambulacral side of 

 a groove on the inner or peristomial face of the ridge which leads 

 to the branchial " cut." 



The protractor muscles and the ligament of the radiales are 

 attached to the " ridges." Each ridge has a thin upper edge 

 and is made up of a single plate comprising the whole of the free 

 edge ; and this is followed in one interradial zone by a low and a 

 moderate-sized plate, and in the other zone by two plates, the 

 first of which is larger than the corresponding plate of the opposite 

 zone. 



There is no median line of suture on the ridge ; and it is 

 evident that this structure is not made on the same lines as the 

 " ridge " of Cidaridse. 



In Mcliinus, when 5 millim. in diameter, the processes are mere 

 nodules, and each is situated on the inner surface of the first 

 ambulacral plate and between the first pair of pores and the inter- 

 radial suture. It is therefore ambulacral, and is not homologous 

 with the projections noticed by J. Miiller on the interporiferous 

 zones of Cidaris. With growth, the poriferous zones of the first 

 six or seven plates become implicated in the mass of a process. 



The "ridge " consists in the young and old forms of a single 

 plate at the edge, and thus it differs from the ridge of Cidaris, 

 w^hich is made up of two plates in one zone, and one in 

 the other. In Fsammechinus more ambulacral plates enter into 

 the structure of a process than in Echinus ; aud on fracturing 

 a process moderately high up, canals are seen continuous with 

 pores. 



The "processes" of the Echinometradee and Diadematidse are 

 on the same plan as those of Echinus, and the "ridges '' differ 

 materially. 



The ridges are wide and low, and there is no single plate at 

 the edge as in Echinus and the Temnopleurids, but two or more 

 plates. There is in one zone a plate 1, and in the other a plate 1 

 and part of a plate 2 ; or, a first plate extends beyond the median 



