The Class Ostracophori 573 



might serve as well as evidence, in a general way, of relationship 

 with Marsipobranchs. Nor is this evidence the more conclu- 

 sive when we reflect that no known type of Gnathostome, recent 

 or fossil, possesses open sensory grooves in distinct dermal plates. 

 The presence, furthermore, of a dorsal fin and a 'truly piscine 

 heterocercal tail,' as noted by Traquair, is by no means as 

 Gnathostome-like as these structures at first glimpse appear. 

 For they lack not merely the characteristic radial supports of 

 fishes, but even actinotrichia. Their mode of support, on the 

 other hand, as Smith Woodward points out, is of a more gener- 

 alized nature, bent scales, homologous with those of the adja- 

 cent body region, taking the place of the piscine external sup- 

 ports." The actual position in the system to be finally as- 

 signed to the Ostracophores is therefore still uncertain. 



Orders of Ostracophores. Four orders of Ostracophori are 

 now usually recognized, known in the systems of Woodward and 

 Traquair as Heterostraci, Osteostraci, Antiarcha, and Anaspida. 

 The former is the most primitive and perhaps the most nearly 

 allied to the sharks, the second is not very remote from it, the 

 last two aberrant in very different directions. Hay places 

 the Antiarcha with the Arthrodira under the superorder of 

 Placodermi. 



Order Heterostraci. The Heterostraci (erepos, different ; ocrrpa- 

 KOS, box) have no bone-corpuscles in the coat of mail. This 

 typically consists of a few pieces above, firmly united and 

 traversed by dermal sense-organs or "lateral lines." The 

 ventral shield is simple. Four families are recognized by 

 Traquair as constituting the Heterostraci, these forming a con- 

 tinuous series from shark-like forms to the carapace-covered 

 Pteraspis. In the most primitive family, the Thelodontidce* 

 the head and trunk are covered with small scales or tubercles 

 of dentine and not fused into large plates. The tail is slender 

 and heterocercal, the caudal fin deeply forked. Until lately 

 these tubercles were regarded as belonging to sharks, and they 

 are still regarded by Traquair as evidence of the affinity of 

 the Heterostraci with the Acanthodei. Dr. Traquair thinks 

 that a flap or lappet-like projection behind the head may be 



* Called CoelolepidcB by Pander and Traquair, but Ccelolepis is a later 

 synonym of Thelodus. 



