74 D. J. SCOURFIELD ON AN ALONA AND A PLEUROXUS NEW TO BRITAIN. 



reason for doubting the correctness of the identification. The 

 species is a typically American form, and I do not think it has 

 been previously recorded on this side of the Atlantic. It may, 

 therefore, have been recently introduced, and it will be interesting 

 to watch whether it succeeds in establishing itself in this 

 country. 



The Devonshire specimen is shown in side view in Fig. 5. In 

 outline it will be seen that it is very similar to P. aduncus and 

 P. trigonellus ; but it differs from those forms by the presence on 

 the shell of evident striae running approximately parallel to the 

 margin on the dorsal part of the head-shield and valves, and also 

 radiating from a central area of the latter to the posterior and 

 ventral margins. Occasionally the striae anastomose, but they 

 never form anything approaching polygonal markings. On the 

 whole, the markings are somewhat similar to those on P. striatus. 

 As is usual in this genus, the anterior ventral margins of the 

 valves are provided with little teeth at the bases of the plumose 

 fringing setae. A highly magnified view of these is given in 

 Fig. 7. At the posterior ventral angle there are usually three 

 little teeth; but the number is subject to some variation, even the 

 two sides of the same individual being sometimes different in this 

 respect, as shown in Fig. 6. 



The post -abdomen (Fig. 9) is quite characteristic. It differs 

 from what is found in the majority of Pleuroxids in being 

 concave on the dorsal edge instead of straight or slightly convex, 

 although this is a character which is shared with P. laevis and 

 P. striatus. The post-abdomen in P. denticulatus, however, is 

 much broader than in the two last-named species, and it is more 

 decisively truncated at the end, thus forming a sharply marked 

 posterior dorsal angle. The armature also is peculiar, for the 

 first two or three teeth — i.e. those at the posterior dorsal angle — 

 are decidedly longer than the rest. Altogether, there are about 

 ten or eleven teeth on each side ; but, as is commonly the case in 

 this genus, the teeth of the two rows do not coincide, and, being 



