138 Dr. Scouler on some fossil Crustacea [Feb. 



tail could not be ascertained. Length of the specimen 

 4J inches. Length of the shell 3 inches. Breadth 1J inch. 

 Length of the tail about lj inch. 



This specimen as will be seen by an inspection of the 

 figure is greatly distorted, the shell has been curved, and 

 the tail or abdomen almost separated from the body. The 

 species is, however, completely distinct from the preceding. 

 The three processes at the anterior extremity of the shell, 

 and the single ridge running along its middle, are suffi- 

 cient to distinguish it. 



That these animals were crustaceous and belonged to the 

 division entomostraca, is sufficiently apparent from a mere 

 inspection of the figures. That they do not belong to any 

 genus at present existing, may also be admitted. It is true, 

 that the characters not only of the genus, but also of the 

 higher groups of crustaceous animals, are chiefly taken 

 from the arrangement and number of masticatory organs, 

 feet and antennae, and that all these parts are wanting in 

 our specimens. Still we have sufficient data for distin- 

 guishing them from all the genera hitherto described. 



In the Limuli the tail consists of a single ensiform appen- 

 dix, and the shell is divided into two distinct sections, 

 characters which the fossil species do not possess. Our 

 fossil species belong to Latreille's class of Branchiopodes 

 which comprehends the genera Apus, Cyclops, &c, but as 

 these genera are distinguished by the number of eyes, we 

 cannot apply these characters to the fossils where every 

 vestige of such parts is lost. Our specimens are, however, 

 nearly allied to the two genera which have been mentioned. 

 They differ from Cyclops, in having three caudal appendices, 

 while in that species there are but two. In the fossil genus 

 the shell consists of a single piece, while that of Cyclops is 

 composed of several sections. In the genus Apus the 

 middle caudal appendix is much shorter than the lateral 

 ones, and of a different form, while in the fossil the three 

 setae are all firm, and the skull has also a different form in 

 the two genera. We may, therefore, consider these ento- 

 mostraca, as constituting a new genus, which we have 

 named Argas, in conformity with the terms, Cyclops, 

 Monoculus, and Polyphemus. 



Entomostraca have hitherto been considered of rare oc- 



