iSo Evolution and Distribution of Fishes 



stome affinity, since that period fits in well as the climax 

 period for cyclostomes. (6) The tissue relation of metane- 

 mertean proboscis teeth, of conodonts, and of cyclostome 

 teeth, suggest a likely evolutionary continuity. Till much 

 stronger evidence has been adduced therefore, than any 

 hitherto advanced for their annelidan or crustacean origin 

 the writer would decidedly favor the view that conodonts 

 indicate the former existence of cyclostome fishes in prodigi- 

 ous numbers, which reached a climax in Mid or late Silurian 

 and early Devonian time. It may be hoped however, either 

 that more definite light will be shed by further study of 

 conodonts, or that some layer of subaquatic volcanic ash 

 may yet be discovered, in which as with the medusae, the 

 annelids, and the skates of the Solenhofen slates, fossilized 

 cyclostomes may be discovered. Though we need not press 

 the point too far, it is worthy of note that the area over 

 which conodonts have been found, namely from W. Russia 

 across Britain to eastern North America, is that which in- 

 cludes the habitat for nearly all of the primitive fishes 

 next to be studied. 



In the above scheme of classification, we next include 

 that ancient and remarkable type from the Lower Old 

 Red rocks of Northern Scotland, which Traquair named 

 Palaeospondylus (Fig. lo, p. 121), It exhibits many of the 

 primitive characters of the Cyclostomata, along with de- 

 cidedly more advanced structural details. The absence of 

 scales of true or even of horny teeth, of limbs and calcified 

 parts, except for the vertebral and neural elements, causes 

 the writer to regard it as a small but advanced sideline of 

 evolution, from a cyclostome ancestry. 



We next reach a large series which the writer would 

 consider to be derivative from that line of metanemertean 

 descent in which secretion of isolated calcareous granules 

 (p. 63), knobs, and ultimately complex surface plates 

 became a hereditary peculiarity. In process of evolution 

 also these slowly split apart into two fairly distinct types, 

 the placoid or isolated, and the lepidoid or overlapping. 

 These accordingly are termed the Placodermata and the 

 Lepidodermata in the foregoing scheme of classification. 

 The former of these on many grounds is generally regarded 



