PvEClLOPODA. 265 



rostrum*, and at others concealed or but slightly visible, fulfils the 

 functions of a mouth. There are never more than fourteen feet. 

 The shell is very thin and composed of a single piece. They are all 

 parasitical. 



AVe will divide this family into two tribes. 



The first — Caligides, Lat. — is characterized by the presence of a 

 shell resembling an oval or semi-lunar shield; by the number of 

 visible feet, which is always twelve, — or fourteen, if we include those 

 which Leach considers as svich, and which I call inferior antennae ; 

 by the form and size of the tenth pairs which are sometimes multifid, 

 pinnate, or terminated in a fin, and well adapted at all times, and in 

 the adult, for the purposes of natation, and sometimes foliaceous, or 

 broad and membranous. The sides of the thorax are never furnished 

 with Aving-like expansions directed backwards and inclosing the 

 body posteriorly. 



Here, the body, exhibiting several segments above, is elongated 

 and narrowed posteriorly, terminating in a kind of tail with two 

 threads, or as many other salient appendages at the end ; this extre- 

 mity is not covered by a segment of the superior teguments in the 

 form of a large rounded scale, deeply notched in the posterior margin. 

 The shell is at least half the length of the body. This subdivision 

 will comprise two genera of Miiller. 



Argulus, Mull. 



This genus was at first designated under the name of Osolus, and 

 but very imperfectly described. Jurine, Jun., has since studied its 

 type with the most scrupulous attention, followed it throughout all 

 its changes of age, and produced a perfect and complete monograph 

 of it. He has restored to the genus the original name given by 

 Miiller. 



The Arguli are furnished with an oval shield, posteriorly emargi- 

 nated, covering the body, the posterior extremity of the abdomen 

 excepted, and bearing on a mediate, triangular space distinguished 

 by the name of clypeus, two eyes, four very small, almost cylindrical 

 antennae placed in front, the superior of Avhich, shorter and triarti- 

 culated, have a stout, edentated and recurved hook at their base ; and 

 the inferior quadriarticulated, with a small tooth on the first joint. 

 The siphon is directed forwards. There are twelve feet. The two 

 first terminate in a transversely annulateddisk, striated and edentated 

 along the margin, and presenting internally a sort of rosette formed 



* The composition of this rostrum or beak is not well known. It is evident, 

 from the figure of the Argulus foliaceus, given by Jurine, Jun., that it contains a 

 sucker ; but is this the case with the others, and of how many pieces is it composed ? 

 I connot answer the question. I presume, however, that this siphon consists of the 

 liibrum, mandibles and the ligula which forms the sheath of the sucker. In the 

 preceding Entomostraca, the four anterior feet, whose form is very different from 

 that of the following ones, would correspond to the four jaws of the Dccapoda. 



