PITH OF THE PERIODICALS. 195 



First family. — Myteloides. These have only two eyes, 

 which are situate close together, and borne by a head which is 

 perfectly distinct from the case. The case is in the form of a 

 bivalve shell, capable of enclosing the body. It has four an- 

 tennae, of which the exterior pair are very large, each composed 

 of an eight-jointed peduncle, and two setaceous branches of 

 twelve joints each. Below the inflated, armed and truncate 

 mandibles, is a pair of foliaceous maxillae. The body is divided 

 into twenty-three segments, each, with the exception of the 

 last, bearing a pair of similar feet, bifid at the end, with the 

 exterior division simply ciliated internally, at the other quadri- 

 articulate, and strongly ciliated exteriorly. The ovaries extend 

 under the sides of the body, from the first pair of feet to the 

 eighteenth, and the eggs, some time prior to oviposition, pass 

 into the dorsal cavity, as into a matrix, where they finish their 

 development. At first they are round and transparent, but they 

 finally become more obscure, and assume an irregular and an- 

 gular shape. Genus, Limnadia. 



Second family. — Aspidiphora. The body stripped of its 

 case is at first almost cylindrical, above convex, below concave, 

 with a central longitudinal furrow, and terminated by an elon- 

 gate cone. It is composed of a head, and thirty or more rings, 

 which diminish greatly in size towards the posterior extremity, 

 and of which the last five or seven are without feet. Genus, 

 Apus. 



Third family. — Ceratophthalma. Of which Latreille has 

 given no general characters, but described the genera which 

 appear to differ very considerably from each other. Genera, 

 Eulimene, Artemia, Branchipus. 



Order. — Xyphosura, 



Being the first edentate order of Crustacea. There is no 

 siphon; maxillae formed by a maxilliform and spinous prolonga- 

 tion of the internal and superior extremities of the coxae of the 

 six anterior pair of legs, and surrounding the pharynx : the case 

 of two pieces ; the anterior large and semilunar, having two 

 compound eyes on its superior surface, the second piece of the 

 case is much less, and in shape is somewhat triangular, trun- 

 cate, and notched at its posterior extremity; to this second piece 

 is appended a sharp, solid, triangular tail. Genus, Limulus. 



