422 



Reviews — Etheridge and Carpenter- 



" Class Blastoidea. — Armless Pelmatozoa of a pyriform, clavate, 

 ovate, or globose shape, which usually exhibit a very perfect radial 

 S3 7 muaetry. Base monocyclic, of two large plates and one small one, 

 the latter being always in the left anterior interradius, Fig. 2. II. (A-B). 

 Five radials, more or less deeply incised by the ambulacra, and five 

 interradials which rest on them and bound the peristome, one of them 

 being pierced by the anus. For view of summit, see Fig.l,B.p.421. 



Ambulacra fringed on each side by a single or double row of 

 jointed appendages, which are in close relation to the side plates. 

 These rest on or against a subambulacral lancet-plate, which is 

 pierced by a canal that lodged the water- vessel and unites with its 

 fellows into a circum-oral ring. 



Hydrospires arranged in ten (or rarely eight) groups which are 

 limited to the radial and interradial plates ; their slits are parallel 

 to, and more or less completely concealed by, the ambulacra, often 

 opening externally through pores at their sides, and also by five or 

 ten openings round the peristome. Neither hydrospires nor ambu- 

 lacra extend below the basiradial suture. 



Peristome naturally concealed by a vault of small plates, which 

 rarely exhibit any definite arrangement, and are continuous with 

 the covering-plates of the ambulacra." 



From the foregoing definition it will be seen that the Blastoids 

 differ in certain important points from other Pelmatozoa, notably in 

 the perforation of the lancet-plate, "of which we have as yet no 

 knowledge whatever in either Crinoids or Cystids. The absence of 

 under-basal plates and the constant presence of five interradials 

 (one of which is divided in Elceacrinus) , lastly the constant but 

 peculiar trimerous symmetry of the base, only observed in the rare 

 I. II. 



,d d 



A7- 



• CLnus 



Fig. 2. — Diagram showing the arrangement of the basals in I. Platycrinus, and II. in 

 the Blastoids. A, B, C, D, U, the five radii of the calyx, x, the small azygos basal. 

 y, z, the two larger basals. d . . . . d, the dorsal axis, r . . . r, the radial axis, 

 in which the anus is situated. The arrows show the (probable) direction of the 

 spirally-coiled digestive tube. 



