1825.] Mr, Gray on the Sea Eggs. 425 



horizontal pentagonal pieces. These bands are placed symme- 

 trically in pairs united together by a flexous suture, the project- 

 ing angle of one series being fitted into the concave angles of 

 the other. The pairs of bands are united together by a strait 

 suture. They are alternately broad and narrow. The broad ones are 

 formed of a few plates, and always imperforated, and the outer 

 edges of the narrow bands, which consist of very numerous nar- 

 row pieces, are perforated by two or more series of minute per- 

 forations placed in pairs ; these perforations form bands, which 

 Linnseus compared to the walks in a garden, calling them ambu- 

 lacra or walks, and the tubercular parts areoij pulvilli, or beds. 

 As in describing the species, it is often necessary to distinguish 

 the character of each of the beds, those of the broad bands 

 might be called extra, and those of the narrow bands intra amhu- 

 lacral beds. 



The vent is surrounded by numerous small scale-like pieces 

 attached to the skin ; these are again surrounded by two series 

 of plates, each formed of five pieces, which are affixed to the 

 body of the crustaceous skeleton. The series of these plates 

 which is next the body, are the smallest ; they are placed just at 

 the top of the ambulacra, and each is perforated with a minute 

 hole, the use of which is quite unknown. The inner series is 

 formed of larger pieces, each perforated with a considerable 

 foramen, which lead to the ovaria. The latter may, therefore, 

 be called the ovarial, and the former, as they are partly 

 between them, the interovarial pieces. One of the ovarial plates 

 is considerably larger than the rest, convex externally, and per- 

 forated like a sieve with numerous minute foramina, and hiter- 

 nally thick and rugose. This plate is somewhat similar both in 

 form, and perhaps in use, to the orbicular spot on the back of 

 the Stellerida called Corpus Spongiosum, by Spix, figured in the 

 Annals of the Museum (vol. xiii. t. 32, f. 1, a), where M. Spix 

 considers that it may, perhaps, be the orifice of the organs of 

 generation as it is perforated with two foramina (see p. 446). 



The skin round the mouth is scaly, and furnished with ten 

 somewhat prominent glands, placed in pairs ; the jaws, which 

 were compared by Aristotle to a lantern, consist of five conical 

 triangular bones, each formed of two pieces, containing in their 

 middle a long linear curved tooth ; the teeth are externally con- 

 vex, and furnished with an internal mid-rib, and the end hardens 

 as they are worn away ; these jaws are articulated together by the 

 intervention of five oblong bones, converging towards the centre, 

 and furnished with five other linear arched bones. The jaws 

 are moved by muscles placed between them, and by some 

 attached to five variously formed erect processes placed on the 

 oral edge of the body of the shell ; these parts are figured, but 

 not very correctly, by Klein, t. 21. 



Round the oral edge of the body of the shell are placed ten 



