228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



them keep their balance and to steer the body ; these fishes are there- 

 fore better adapted to swimming than are the young tadpoles. 



After thus having described the four most important types of 

 adaptations to swimming, we will now consider the modifications of 

 the various old Paleozoic fishes and fishlike forms and try to deter- 

 mine to which of these four types their various representatives have 

 belonged. 



We will start with the first class, that of the Ostracoderms, which 

 is divided into two subclasses — Cephalaspidomorphi and Ptera- 

 spidomorphi (fig. 1). 



The Cephalaspidomorphi are known from the Upper Silurian and 

 throughout the Devonian. And we can here differentiate two 

 sharply defined orders; the Osteostraci and the Anaspida. 



The Osteostraci, which are particularly well known thanks to the 

 recent investigations by Stensio, begin to appear in the Upper 

 Silurian, in which they are represented by some curious flat forms 

 shown in the upper left of our figure 3. This picture shows the 

 so-called Tre7rubtaspis from the Upper Silurian of Estonia. 



The head and anterior parts of the body are covered by a con- 

 tinuous shell. On the dorsal side we find two close-set eyes (fig. 

 3) with a pineal organ between them and a single nostril in 

 front. At the sides of the shell there are some peculiar sense organs, 

 which Stensio interprets as electric organs. On the ventral side 

 (fig. 3) there is a large opening anteriorly, the so-called mouth- 

 gill opening or oralo-branchial aperture, which is covered only by 

 smaller plates. In this area we find the mouth anteriorly and the 

 gill-openings posteriorly on the sides. The tail is small and thin, 

 covered with large scales, and distinctly triangular in cross-section 

 (fig. 3). The flat belly forms the underside of the triangle. 

 Where the sides join there is a peculiar comblike row of scales 

 forming three longitudinal fringes. These rows of scales are prob- 

 ably the first indications of the medio-dorsal and lateral fin folds. 

 The caudal fin itself is comparatively small, heterocercal, that is 

 with the upper lobe larger than the lower, and is covered with scales. 

 How then has this creature been able to move through the water? 



A glance at the reconstruction of this form is sufiicient to show 

 a striking similarity in the shape of Tremataspis and that of a 

 tadpole. Both have a plump, rounded cephalothorax and a thin 

 rather flat tail. Both are without paired fins and have no well- 

 developed organs of equilibrium. We can consequently say with 

 certainty that Treniataspis was a bottom form, swimming only 

 poorly and uncertainly. Its organs of propulsion, balance, and 

 steering are, as in the case of the tadpole, all represented only by 

 the tail and the posterior part of the trunk. But it is nevertheless 



