230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



structure of the tail was probably similar to that in Tremataspis 

 but broader. In this form we, therefore, have an addition to the 

 three longitudinal folds on the posterior part of the body in the 

 form of the two flat projections on the sides of the cephalothorax, 

 which serve as organs of equilibrium and at the same time increase 

 the surface of the cephalothorax and thus function also as an 

 accessory gliding surface. 



The most highly specialized forms of the Osteostraci are the 

 Cephalaspids (fig. 3) which are still better adapted to swimming. 

 In these forms, practically only the head is covered by a hard shell, 

 while the entire rest of the body has scales only and is therefore 

 movable. The tail is large and the median and lateral folds are 

 more strongly developed. In addition, most of these forms have 

 a distinctly developed dorsal fin. 



The projections on the side of the head already indicated in the 

 earlier forms have become larger — some even very large. On the 

 posterior part of the head a third median dorsal projection is dis- 

 tinctly developed and may reach a large size in some species. We 

 finally see in the bights between the lateral projections two peculiar 

 lobe-shaped organs of the nature of paired, movable pectoral fins. 



All this seemed to indicate that the Cephalaspids were considerably 

 better swimmers than Trematas'pis^ and they are no longer so 

 tadpolelike but approach the true fish type more closely. The tail 

 is still the most important organ of propulsion, and the lateral 

 and median folds, the dorsal fin on the body, and the lateral and 

 median projections on the head are excellent organs of equilibrium. 

 We finally have the paired pectoral fins as effective organs of 

 swimming. Even if C ephalaspis was a typical bottom form, it 

 could certainly swim as well as, for instance, one of our recent 

 sculpins. 



We also see very clearly how the different Osteostraci became grad- 

 ually better and better adapted to swimming. Parallel with this 

 development a gradual reduction in the thickness of the external 

 skeleton also took place. In the oldest form such as Treviatas'pis the 

 shell is thicker, in the younger forms it is more and more strongly 

 reduced as particularly pointed out by Stensio. With this reduction 

 the fish becomes lighter and in consequence better fitted for swim- 

 ming. The second group of the Cephalaspidomorphi (fig. 1) are 

 the so-called Anaspids, which first became really known after the 

 thorough investigation of recent finds published by Kiaer. The 

 Anaspids (fig. 4) are only known with certainty from the upper 

 Silurian, but some uncertain remains have also been described from 

 the Upper Devonian. They have a strongly compressed torpedo- 

 shaped body with a pointed snout and a powerful so-called hypocer- 



