20 REPORT — 1855. 



Thus it will be perceived, that, instead of considering Trilobita, Entomo- 

 straca, and Rotatoria as orders belonging to the second division of Crustacea, 

 as Dana has done, we take them to form natural divisions in themselves, 

 with wider structural demarcations than exist between the Macroura of the 

 first division and the Amphipoda of the second. This nearer approximates 

 the system of arrangement adopted by Milne-Edwards in his ' Histoire des 

 Crustaces.' But in his classification, Latreille's order of LcBmodipoda is 

 admitted to a rank of equal importance to that of the Amphipoda or 

 Isopoda. 



This, from a correct appreciation of the homological relation of the several 

 parts, Mr. Dana (whom as a carcinologist no one appears to have surpassed 

 in close observation) entirely ignores, and embraces the Lcsmodipoda within 

 the order of true Amphipoda, making no allowance in his arrangement for 

 their naturally aberrant departure in outward form from that group. " They 

 are," says that author, " properly therefore Amphipoda with certain parts 

 obsolescent. . . . The more essential characters are closely related to the 

 Amphipoda rather than to the Isopoda, and are not properly intermediate, 

 nor a new type alike distinct from both." — Vol. i. p. 11. 



This author, while from anatomical reasoning. he removes the Lcemodi- 

 poda from the position in which they have been placed as a separate and 

 intermediate order between the Ainphipoda and the Isopoda, yet sees in 

 another group, which by every previous naturalist has been ranked with 

 Isopoda, a "true intermediate species between the Amphipoda and Isopoda; 

 and if any third or intermediate group be admitted, these should (he thinks) 

 be considei'ed as constituting it. These species belong to the genera Tanais, 

 Arcturus, Leachia, and others allied." — Vol. i. p. 11. These form the tribe 

 or group of Anisopoda, the second or intermediate of that author. 



By the force of similar arguments as those which are employed for the 

 removal of the Lcemodipoda from taking a position distinct from the Amphi- 

 poda, it is difficult to imagine that so acute an observer as the founder of 

 this new group should separate it from tlie true Isopoda upon grounds so 

 feeble as appears to us to be the case. 



But on this we shall enter more at large when we report upon the British 

 Isopoda, and at present only observe, that the affinity which the Anisopoda 

 holds to the true Isopoda in all its more important characters is too close 

 to admit of its being recognized as a distinct and separate group of equal 

 importance. The only feature which, appears to approximate it to the 

 Amphipoda, the forward direction of the fourtli pair of feet, can scarcely, 

 we think, be of sufficient importance to narrow the margin between the 

 Amphipoda and the Isopoda, thei'e being other characters of greater import- 

 ance that induce a natural separation strongly marked. 



But although anatomical science will not admit the elevation of the 

 Lcemodipoda or that of the Anisopoda into distinct orders or groups equal 

 to that of the Amphipoda and Isopoda, yet the presence of strongly defined 

 characters, both in development of form and suppression of parts, might 

 safely admit, with great convenience to classification, a separation of the 

 Lcemodipoda from the Amphipoda proper, and the Anisopoda from the IsO' 

 poda proper, each forming a group subordinate to their respective types ; 

 and in this Report we propose the following arrangement : — 



