ON THE OCCURRENCE OF AN APUS 315 



downward at the cervical fold, where as in the recent form they 

 open (at the underside of the body). 



The median line of the carapace is marked in its anterior third 

 behind the cervical fold by a deep depression, fading out backward, 

 which corresponds to the carina seen in A pus cancriformis and 

 some of its congeners. 



The posterior emargination is distinct in both specimens though 

 not appearing in the photographs. The original of Figure 2 is 

 just sufficiently compressed obliquely to have the emargination 

 transferred to the other side. 



General hearing of discovery. — ^Apus has long been famous in 

 paleontologic literature as a primitive phyllopod that on account 

 of its great number of simple appendages and other primitive 

 features has served well as a model for comparison with extinct 

 crustaceans, especially with the trilobites; and again in the case 

 of the wonderful middle Cambrian branchiopods discovered by 

 Walcott in British Columbia. It is equally famous among zoolo- 

 gists for its strange life-cycle as well as its archaic characters. 

 Notwithstanding its frequent citation in paleontologic literature, a 

 true Apus has only once been found in fossil state. This is the 

 Apus antiquus Schimper from the Buntsandstein of the Vogesian 

 mountains. This find carried the range of Apus back to the 

 Triassic, and the occurrence in Oklahoma extends it now to the 

 Permian. 



Apus is thereby made one of the few persistent types that 

 have existed from Paleozoic to recent time. Like Limulus it is 

 a "living fossil." Connected with this amazing persistence is 

 undoubtedly the strange life-cycle of this creature, as the writer 

 will elaborate more fully in another paper. Apus, as typically 

 represented by Apus cancriformis, appears only at long intervals, 

 usually only after decades of years, during which the eggs were 

 buried in the dry mud of roads, ditches, and desiccated pools and 

 exposed to heat and cold. It is therefore an extremely rare animal 

 and the writer still remembers with thrill, how when still a school- 

 boy he one day espied a large specimen crawling on the muddy 

 bottom in the water of a swamp along which he was botanizing, 



