196 pith of the periodicals. 



Order. — Siphonostoma. 



These have a siphon, or sucker, more or less distinct, 

 formed of four pieces, corresponding to the labrum, tongue, 

 and two mandibles, of the dentate Crustacea. Number of 

 feet never exceeding fourteen ; case composed of a single 

 piece, forming in front a kind of buckler. 



First family. — Caligides. These have many of their feet, 

 more especially the posterior ones, formed for swimming. 

 The posterior extremity of the body is preceded by a shield- 

 like or semi-lunar case. Genera, Aryulus, Caligus, Pan- 

 darus, Pterygopoda, Dinemoura, Anthosoma, Cecrops. 



Second family. — Lernceiformes. Their body is elongate, 

 cylindrical, attenuated postei'iorly, composed of from seven 

 to ten segments. The feet are small, or the analogues of the 

 natatory feet in the preceding order are composed of but a 

 single articulation, and two fingers, or two minute articulated 

 stalks ; sometimes the sides of the thorax are dilated into 

 large lobes, bent in the form of a horse-shoe, and embracing 

 the posterior portion of the body : they have at least two 

 antennas, and the feet are furnished with claws. Genus, 

 Nicothoe (found in branchias of the lobster), Dichelestium 

 (found on the sturgeon), Nemesis. 



Order. — Trilobites. 



This singular order of Crustacea, apparently belonging to 

 another and an older creation, are, perhaps, of all animals 

 the most difficult to place naturally. They at first sight 

 appear to supply a void which occurs between the Crustacea 

 and the genus Glomeris, at the commencement of the Myria- 

 poda. Genera, Calymenes, Asaphus, Oxygya, Para- 

 d ox ides. 



(To be continued.) 



