PEEIGNATHIC GIKDLE OF THE ECHIKOTBEA. 208 



About nine minor plates have their pores on tlie inner flank of 

 a process ; and none of these plates have their poriferous part 

 limited by suture, and the suture between the ridge and the pro- 

 cess is remote from the pores. But all succeedi^ig plates have 

 their poriferous zones in contact with the closely placed ambu- 

 lacro-interradial suture. 



VI. The Suborder Cltpeastrid^, &c. 



Students of the Echinoidea are under great obligations to A. 

 Agassiz for his revision of the genera and his maguificent plates. 



His descriptions and illustrations of the Clypeastroids are 

 especially excellent, and the drawings and photographs which 

 represent the internal structure of the species are admirable. 



The nature of the jaws of the Clypeastroids will be found in 

 most works on the Echinoidea, and all that is necessary to be 

 mentioned here is to follow Agassiz and state that " The mode 

 of articulation of the jaws upon the auricles is entirely distinct 

 in the Clypeastroids and in the Desmosticha ; in the Clypeastroids 

 the auricles are disconnected, and when the jaws are in position 

 they completely hide the auricles on which they ride. The mus- 

 cular system of the jaws of Clypeastroids is reduced to a very 

 feeble band attached to the underside of the pyramids, and ex- 

 tending to the auricles " (Eev. Ech. p. 689). The figures of 

 Clypeaster subdepressus (pls.xxx. and xi. h), Ecliinantlins rosaceus 

 (pi. xxviii.), Olypeaster scutiformis (pi. xiii./"), and EcMnodiscus 

 auritus (pi. xiii. c), &c., show the position of the structures, the 

 muscular fibres being omitted. 



I have been able to dissect a specimen of Laganum depressum 

 which contained the viscera, and I have had the advantage of 

 studying specimens of Clypeaster {Uchinanthus, A. Ag.) rosaceus 

 and Clypeaster humilis at the British Museum. 



It is quite evident that these three species are not formed upon 

 the same type as regards the supports of the jaws. There is an 

 interesting difference which should be of classificatory value ; 

 for whilst in both the forms of Clypeaster there are two pro- 

 cesses supporting a pyramid, in Laganum and also in Echinara- 

 chnius, Mellifa, and EcMnodiscus there is but one support to the 

 fifth part of the whole jaw-apparatus. 



Unfortunately all the other genera of the suborder have not 

 been at ray command ; nevertheless, by taking the two forms of 



