MUSEUM OF COMPARATIVE ZOOLOGY. 285 



Psammechinus, Trigonoeidaris, appearing between the ten plates and 

 the test. This mode of growth is totally unlike the growth of the 

 buccal plates of the Cidaridae, where these plates perform the part of 

 ambulacral and interambulacral plates, and appear near the test at first, 

 forming in full-grown specimens rows made up of more than two plates, 

 a> in the Palaechinidae, suggesting that the test of Palaechinidae must have 

 been made up of plates homologous to the buccal plates of Cidaris. 

 The test of course would then have been capable of considerable com- 

 pression and change of outline, as is the case in Astropyga and Astero- 

 soma. This similarity is very striking in young Cidaridae, where the 

 number of coronal plates is small, and the young Sea-urchin seems to 

 consist almost entirely of an abactinal and an actinal system, separated 

 by a narrow band of coronal plates. Let this narrow band of coronal 

 plates disappear entirely, and the buccal plates take a correspondingly 

 great development, and we have a Palaechinus made up of small ambu- 

 lacral and interambulacral plates consisting of several rows, and con- 

 tinuous from the teeth to the abactinal system, similar to that discovered 

 by Meek and Worthen, the whole test surmounted by short spines, 

 articulating upon a more or less distinct mamelon. The structural 

 features of the buccal membrane of Cidaridae entitle them to a higher 

 rank than that of a family, in the suborder of Echinoids, intermediate 

 between the Palaechinidae and Echinidae proper. 



In the Temnopleuridoe (Toreumatica) the subanal plate remains very 

 prominent in adult specimens ; the anal system in the young is covered 

 by one large elliptical plate ; as the anal system enlarges, numerous 

 minute plates surround the larger plate, which always retains its 

 peculiar ornamentation, and is readily distinguished from the other by 

 its size and shape. In Temnotrema, on the contrary, the anal system 

 undergoes changes identical with those of Toxopneustes, Echinus, and 

 the like. In Toreumatica, the pits at the angles of the plates appear at 

 first like rectangular openings, which, as the specimens grow older, be- 

 come little by little connected by grooves, growing deeper and more 

 prominent w T ith advancing age. The same is the case in Temnotrema; 

 the pits, however, are never so marked in the adult, becoming simply 

 comma-shaped. The miliaries in both these genera are formed as in 

 other genera by ridges appearing at first connected with the base of the 

 primary tubercles. In Trigonoeidaris the young differ from the old in 

 having larger pits, less numerous and lower ridges, and but few sec- 



