ON THE OCCURRENCE OF AN APUS 313 



that the term Apus expresses here the hard parts only and the entire 

 group (Apus and Lepidurus) and that it is merely intended to 

 point out the persistence of the Apus-type of carapace. The 

 persistence of the exact form of the carapace in the group indicates 

 that no evolutionary development profound enough to affect the 

 carapace took place in all this time. 



Note on preservation of material. — The specimens are casts of 

 the interior surface of the carapace. It is for this reason that the 

 impressions of the shell-glands are so well preserved in some of the 

 fossils, for it must be remembered that these organs are situated 

 between the two layers that form the posterior portion of the 



Fig. 3 ' Fig. 4 



Fig. 3. — Apus beedei. Flat projection of outline of carapace. 



Fig. 4. — Lateral view of Apus aequaUs Packard, X2; from Colorado and in 

 N.Y. State Museum. 



carapace, and that these glands open where the inner layer termi- 

 nates at the cervical fold. The frontal portion of the carapace is 

 poorly preserved, partly no doubt on account of the greater thin- 

 ness of the carapace there, which consists of but one layer, and 

 partly owing to the lateral compression of the tests during entomb- 

 ment. There is, however, enough left of the frontal outline 

 (especially in the smaller specimen) to leave no doubt that it was 

 rounded and unbroken originally. It would seem that the shield 

 was sufficiently sloping toward the lateral margins to come fre- 

 quently to rest on the side and then suffer lateral compression. 

 The amount of lateral slope is greater also in the recent Apus than 

 the usual dorsal views of the creatures would suggest, as shown in 

 the lateral view drawn from nature by the writer and reproduced in 

 Figure 4. A restoration of the dorsal view of the carapace of 

 Apus beedei is given in Figure 3. It is obtained by plotting the 



