BIHANU TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. NIO 1. 9 



contrasts with tliat of the pyramid'. At tlie aboral eiid 

 of each half-pyramid is seeii laterally a small aggregation 

 of coarser reticnlar tissue, the commencement of the epiphy.si8 

 Johannes Muller, Stewart (arcus Valentin), and in the middle 

 between every two pyramids a still more simple rudiment 

 probably of the rotula Desmoulins, Joh. Muller (falces Val., 

 8tew.; braees Al. Ag.). The inter-pyramidal muscles between 

 every two pyramids could not be perceived. The pairs of ex- 

 ternal motor muscles for each of these, the two retractors and 

 tlie two protractors, inserted iipon the two half-pj^ramids, all 

 arise from the solitary interradial plate, the two protrac- 

 tors, »1. pro., very wide apart, the retractors, m. re., very near 

 theni. It will be seen furtlier on that during the growth of 

 the test the attachments of these four muscles are removed to 

 paired interradials and finally disposed in another manner. 



The fig. 28 is a side view of the dental apparatus in 

 another somewhat more advanced specimen. It is detached 

 from the buccal membrane and from the peristome. The epi- 

 physes, the rotule and the compasses are forming in their 

 places. The pyramids are now raised to near their future 

 normal all but erect position, their halves have almost closed 

 at their adoral ends, and there already exhibit some indication 

 of their future shape, but upwards their inner margins, which 

 in the adult will be firmly nnited by nieans of a straight 

 and prolonged symphysis, still diverge strongly and leave 

 wide open the foramen magnum Val. Through tliis opening 

 the tooth is seen, of outward shape as in ////. 22 but leng- 

 thened adorally, and with the lameilar structure distinctly 

 delined. The lamels, the general form of which corresponds 

 with that of the tooth, are laid one upon another regularly. 

 from the top downwards. The fig. 24, from a transverse sec- 

 tion of a decalcified specimen slightly more advanced, shows 

 the lameliar structure which seems to exist alone at this pe- 

 riod. 



Thus — assuming it allowable to extend to the wliole 

 group the facts elicited from a single species, and that one 

 exceptional in its development, — the Cidarid, on the eve of 

 entering upon its final mode of existence, enclosed within the 

 pluteal envelope which covers it on all sides, has assumed 



' Krohn loug since described the teetli in a voung astomous Echinus. 

 MlTLL. Arch. 1851, \k 331. 



