206 PROF. p. MARTIN DUNCAN ON THE 



of specimens to be able to state that this distinction is in- 

 variable. 



With regard to the jaws of Clypeaster, it will be remembered 

 that each pyramid has two cavities on its inferior surface, and a 

 process fits into each one and supports the jaws. It is not the 

 two processes of the same ambulacrum which fit into the cavities 

 of the same pyramid, but the process of one ambulacrum and the 

 process of the next ambulacrum which is situated just on the 

 other side of the interradium. So far as I can make out, the 

 muscle starts from the front of a process and reaches a pyramid 

 close above the teeth, and it acts with those of the other pro- 

 cesses as an opener of the jaws. 



Laganum depressum, Lesson. — On dissecting a specimen it is seen 

 that the ambulacra at the actinal surface have a large pore close to 

 the peristomial edge of each first plate, and that the first plates 

 are large and have a median furrow ending in a swelling between 

 the large pores or slightly externally to them. Numerous rows of 

 minute pores start from close to the median line and reacli out- 

 wards until a radiating series of small tubercles, five or six in 

 number, is reached. These tubercles are within the interradium 

 and are on the interradial side of the ambulacro-interradial suture. 

 A corresponding series of tubercles is on the other side of the 

 single interradial plate no. 1. This plate is single and fits in 

 between two ambulacra, and it is sutured to two interradial 

 plates circumferentially. See also Loven, ' Etudes,' plate xlv. 



Now on removing the abactinal part of the test and taking 

 off the jaws, it is at once noticed that the arrangement of the 

 girdle is unlike that seen in Clypeaster. There are only five 

 projections instead of ten, and each is curved, concavity towards 

 the peristome, from which it is separated by a distance equal 

 to its own height, which is not great however (figs. 50 & 51). 

 The posterior projections are larger than the other three and 

 are more pointed (fig. 52). 



Benzene displays, within, the suturing of the plates which were 

 recognized on the surface. The limits of the ambulacral plates 

 are well defined and they are attached to the sides of the inter- 

 radial plate no. 1. The pores may be seen on the surface of the 

 ambulacra within, and also some stout transverse lines ; but it is 

 evident that the projection has not its base within an ambulacral 

 plate. The projections are growths from the upper surface of the 

 first interradial plates, which are single, in each interradium, at 

 the peristome, and they are therefore homologues of " ridges." 



