LIFE IN PRIMARY PERIOD 231 



temporal and occipital scales, like their teeth, are a heritage 

 from the Fishes. 



The preceding considerations retrace in a general way 

 the genealogy of Fishes, and indicate the order of their 

 appearance. Not a single species is known in the Cambrian, 

 and what we know of the Silurian is evidently incomplete. 

 In the Devonian, however, there is a singular form, Palceo- 

 spondylus gunnii, which, if it does not belong to the Marsipo- 

 branchs, belongs to a still more primitive type. Analogous 

 forms must have existed in Silurian times, but they are 

 unknown. On the other hand, in the Upper Silurian of 

 England, the Island of Axel, Podolia, Galicia, Ludlow, and 

 various other places, there are strange creatures which we can 

 only consider as Fishes, but which do not seem to fit into 

 any existing series. Their shape was flat, and they bore 

 a superficial resemblance to the Trilobites, especially on 

 account of the shield-like form of their head. Their large 

 ventral mouth, elongated into a transverse slit, had no jaws, 

 but their cephalic cuirass was protected by real bones, containing 

 bone-cells. They initiated the series of Ostracodermous 

 Fishes devoid of lateral fins. Among them were Cephalaspis 

 and Aiichenaspis, associated with Pteraspis, whose trunk 

 and tail were covered with lozenge-shaped scales, and with 

 various other genera. 1 To these are to be added similar 

 fishes which, however, were provided with a pair of paddle- 

 shaped fins, also covered with polygonal bony plates, 2 and 

 of these certain forms may have existed from Silurian times 

 on the east coast of the Baltic. At the same time other forms 

 appeared, with occasionally globular heads, 3 strongly armour- 

 plated with polygonal bony articulated plates, which gave 

 them a very distinctive appearance. 4 



The fins of Pterichthys are already highly specialized organs. 

 Since many of the Selachians, even during the Carboniferous 

 period, 5 and of which there are representations to-day, 6 have 

 very primitive fins conforming almost to the genealogical 

 indications furnished by embryogeny, it must be assumed 



1 Ateleaspis, Birhenia, Cyathaspis, Lanarkia, Thelodus, etc. 



2 Asterolepis, Bothriolepis, Pteriichthys. 



3 Coccosteus. 



4 Dinichthys, Heterosteus, Homosteus, Titanichthys. 



5 The Pleuracanthidae especially. 



6 Chlamydoselachus of the Japanese waters. 



