138 PKOCEEDINGS OF TKE GEOLOGICAL SOCIETY. [June 23, 



appeared in Mr. Kirkby's drawings*) situated at the extreme limits 

 of the anterior inferior margin of the cephalon. 



In Isopoda (except in Dana's tribe of Anisopoda) the eyes are 

 external to both pairs of antennae, whereas in the Amphipoda they 

 are situated between the upper and lower antennae. 



The mandibles in the Isopoda articulate within the lateral marguis 

 of the cephalon ; but in Amphipoda they articulate upon the lateral 

 margins, posterior to the second pair of antennae. 



Posterior to the cephalon in the specimen (fig. 6) are at most four 

 narrow segments, which are not as deep as the cephalon ; they are 

 laterally compressed and marked by a slight dorsal keel. 



The other specimens consist of four or five narrow segments, and 

 two large ones (one very large and the second nearly as large), 

 situated posteriorly. The narrow segments are not more than half 

 as deep as the larger segments, to each of which, in one or two of 

 the more recently-found specimens, there is attached a small plate : 

 these plates are the coxae of the legs (the ^'epimera" of Edwards). 

 These coxae bear a resemblance to those of the recent Amphipod 

 (Phcedra antiqua) in the annexed plate (PI. YI. fig. 8), In the 

 fossil the second segment of the pleon has the posterior margin 

 ornate in the more perfect specimens, being toothed at the infero- 

 posterior angle, and lobed near the middle, thus possessing a waved 

 and graceful appearance (figs. 3 and 4). 



Another specimen exhibits the commencement of a segment pos- 

 terior to those already given ; and slight indications of this may be 

 seen in fig. 3. 



The fragments that have been found belonging to this portion of 

 the animal are laterally compressed, and, in aU the specimens 

 except one, are surmounted by a slight dorsal ridge. 



That all the specimens are parts of the same Crustacean species, 

 I think may be shown from the corresponding size and the relative 

 depth of the segments of the pereion. The circumstance that one 

 specimen is not carinated will scarcely interfere with this opinion, 

 since it is evidently an exception to the rule, depending upon some 

 peculiarity in the growth of the individual. 



In the development of those Amphipoda that have a dorsal carina, 

 the larva appears first without that distinguishing feature, which is 

 afterwards gradually added. We can easily understand how a slight 

 arrest in the growth of the animal may interfere with this pecu- 

 liarity. Therefore, when an animal in its normal condition possesses 

 a carina so very indistinctly marked as in the fossil, it is not unwise 

 to assume that the absence of that character is but a slight diver- 

 gence from the typical condition, therefore one of variety or sex. 



We therefore come to the conclusion that the specimens belong 

 to one species of animal, and that the animal is an Amphipod 

 Crustacean, from the recent type of which it difiers in the prominent 

 eye and the greater depth of the anterior, as compared with the 

 second, segment of the pleon. 



* Ibid., pi. vii. figs. 1-3. 



