The Primitive F^ishes 261 



as the more primitive, and reasons for accepting such a view 

 are advanced later. We have divided it into six subgroups 

 that are of diverse morphological value. 



From the slight and rather superficial knowledge as 

 yet obtained of the animals, the Protoplacoda (Coelolepi- 

 dae) deserve to be placed lowest in the scale. Of the recog- 

 nised genera Thelodus includes one species that is known 

 only by its isolated placoid scales from the Upper Silurian 

 of W. England, of N. E. Germany and of W. Russia. 

 A second, known in the entire fish, occurs in Lower Old 

 Red rocks of central Scotland (Fig. 8a, p. 112), while a 

 third is from the Upper Devonian of Russia. Lanarkia 

 is found in the "Downtonian" beds of Lanarkshire in 

 Scotland (Fig, 8b, p. 112). Both were provided with scat- 

 tered tubercles {Thelodus) or hollow spines [Lanarkia)^ 

 but were probably of soft cartilaginous tissue within. 

 Mouth and sense-organs are as yet unknown. 



The Heteroplacoda include Drepanaspis and Psammost- 

 eus that occur from Britain eastward to Russia. In contrast 

 to the first group these are covered by flat tubercles or 

 plates of variable size, those covering the head being at 

 times large flat shields with intervening smaller plates. 

 The simple transverse slit-like mouth, the paired lateral 

 sense-organs, and probable soft body substance, suggest 

 sluggish primitive animals derived from evolutionary ad- 

 vance on the Protoplacoda. 



The Symplacoda consist of three genera Cyathaspis, 

 Palaeaspis, and Pteraspis, that are distributed from the 

 Upper Silurian of England and Galicia, to the Lower Old 

 Red Rocks of England, Scotland, Galicia, Eifel, East 

 Prussia, Spitzbergen, and New Brunswick in Canada. 



Regarding the above three genera Bridge (?<5:53o) 

 following Traquair says "that they constitute a natural 

 sequence of forms, beginning with organisms whose Elasmo- 

 branch ancestry is extremely probable." The writer would 

 incline to view the first of the three as a very primitive 

 placoid type, which by progressive specialization gave rise, 

 in very direct line, to the elasmobranch series, and to deriva- 

 tive side-lines that constituted the two latter genera. These 

 had a heavy defensive armature and clumsy habit. 



