234 D. J. SCOUKFIELD ON EPHIPPIA OF LYNCEID ENTOMOSTRACA. 



as long as the total length of the ephippium, is considerably 

 shorter than this, being perhaps not more than half, or even less. 

 The anterior margin also joins the dorsal margin at an angle 

 considerably greater than a right angle, whereas, in all the 

 ephippia hitherto examined, the angle has not differed much from 

 a right angle. These two points, which are of course mutually 

 related, are determined by the great backward extension of the 

 head- shield typically shown in the genera Pleuroxus and Ckijdorus, 

 though it does not by any means follow that all the species at 

 present included in those genera exhibit these characteristics. 

 In Fig. 26 a drawing is given of an ephippium of Peratacantha 

 truncata, with the ventral margins of the valves still attached 

 posteriorly. The ephippium is much darkened, and the dorsal 

 thickening very strong, but the shell markings, always rather 

 vague, do not seem to be much altered. The line of weakness, 

 which can be easily seen in the living ephippial female, is marked 

 by a double or triple row of imperfectly connected pieces of chitin, 

 as indicated in Fig. 26, and more highly magnified in Fig. 27. It 

 reaches the ventral shell margin a little way in front of the 

 posterior ventral angle. The inner membranes, which are much 

 wrinkled and folded, form a capsule round the egg, which is 

 probably entire, but appears rather indefinite posteriorly. The 

 shape of the egg seems characteristic in this species. It is always 

 about as shown in Fig. 26 — that is, the broader end is placed 

 anteriorly, and is somewhat obliquely flattened. Weismann also 

 shows it thus (12, Taf. vii.. Fig. 11b). 



This species is a very good example of the way in which some 

 Lynceids attach their ephippia to the sides of the vessels in which 

 they are kept. In a collection from Richmond Park, made in 

 November, 1898, I counted, after a few days, no less than sixty of 

 these bodies adhering to the sides of a small glass jar. 



Pleuroxus laevis Sars. 



The ephippium of this species, shown in Fig. 28, is of almost 

 exactly the same shape as that of Peratacantha truncata, and 

 resembles the latter closely in nearly all other respects. The 

 ordinary shell sculpture, consisting of very faint polygonal 

 markings, is present all over the ephippium, but the chitin is 

 minutely pitted. The line of weakness is formed in very much 

 the same way as in P. truncata, but there do not seem to be so 



