REPORT OF THE DIRECTOR I916 IIO, 



mud-burrowing trilobites Conolichas, Hoplolichas, and also by the 

 many relicts of extinct groups among the recent burrowing animals, 

 as Amphioxus, the Caeciliae, Limulus, etc. Even among the 

 extremely variable insects we find in the cockroaches, which are 

 given to a secretive life under stones and logs, a group of remark- 

 able persistence of characters which have changed relatively little 

 since Paleozoic time. 



5 A perusal of the percentage table shows further that the 

 sessile forms contain more persistent types than the vagile ben- 

 thos. Thus the Corals have 15 per cent, the Byrozoa 22 per cent, 

 the Cirripedia 20 per cent. Likewise many of the immortal and 

 very long-range persistent types in the other classes are more or 

 less sessile forms, as Crania, Schizocrania, Pholidops, Spirorbis, 

 Ortonia, Serpula, Terebella, Ostrea, Chlamys, Camptonectes, 

 Entolium, Pseudamusium, Platyceras, Capulus, Acmaea, Patella. 



On the other hand, the sessile sponges and Crinoidea show but 

 relatively small percentages of persistent forms. 



In the case of the sponges we are sure that the Paleozoic forms 

 are only very imperfectly known owing to the sporadic occurrence 

 of the sponge fields. Thus the Dictyosponges, which in the New 

 York Upper Devonian have furnished a great variety of forms, 

 are only rarely met with elsewhere (Carboniferous of Mississippi 

 valley, Devonian of France and Poland) and their range is there- 

 fore probably greater than known at present. The Hexactinellid 

 sponges which culminated in the Jurassic and Cretaceous time, 

 have withdrawn into the greater depths of the oceanic basins and 

 through this different habitat undoubtedly been forced to change. 

 The Crinoidea display a great multitude of rapidly developing, 

 short-range forms in their climacteric period in the Carboniferous ; 

 their persistent types do not appear until later. 



It is further noteworthy in this connection that the Rudistae, 

 typically sessile pelecypods, and the Richthofenidae, most remark- 

 ably modified, sessile brachiopods, were extremly short-lived. 

 In both cases we have groups that were widely divergent from 

 the primitive central stock of their classes, and it can be claimed 

 as a general proposition that groups that diverge widely from the 

 median expression of a class do not become persistent. 



A distinct example of the different influence of the sessile and 

 vagile modes of life on the longevity of forms is furnished by th?. 

 graptolites. The writer lias in other places shown the contrast be- 

 tween the Dendroidea which remained sessile to the sea bottom and 



