232 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 193 4 



The second large subclass of the Ostracoderms (fig. 1), the Pter- 

 aspidomorphi, have a more peculiar build. Some of the oldest forms, 

 which are known from as far back as the Ordovician, are the so-called 

 Thelodonti (fig. 5). The entire body including head, tail, and fins, 

 is evenly covered with small, close-set scales. The head and the fore- 

 most part of the body, the " cephalothorax ", is broad and flat ; the 

 hindmost part, on the other hand, is narrow and the tail probably 



Ord. Thelodonti 



TlGCRB 5. 



-Two representatives of the order Thelodonti (after Traquair). 

 from Scotland ; B, Thelodus from Scotland. 



A, Lanaikia 



hypocercal. The postero-lateral corners of the cephalothorax are 

 produced into a pair of flat, brimlike fins. On the posterior part we 

 find a small anal and dorsal fin. In their shape the Thelodonti re- 

 mind one very closely of the oldest Osteostraci which we have just 

 described, and also of the tadpole. We have certainly to deal with 

 bottom forms, which were swimming around like tadpoles even 

 though they had organs of equilibrium in the form of lateral flaps 

 and also a median fin. 



