586 Arthrodires 



pairs of elements. Ventral armoring of two pairs of lateral 

 and two median elements." 



Temnothoraci (re^vco, to cut; Qopag, thorax). "Arthrodires 

 whose cranial and dorsal shields are closely apposed, separated 

 only by a transverse fissure-like interval (whence the ordinal 

 name) ; interarticulation of cranial and dorsal shields little 

 developed. Head-shield elliptical in outline as far as the line of 

 the transverse division. The anterior rim of the shoulder-shield 

 flattened at its sides, suggesting a rudiment of the vertical 

 partition of the Anarthrodira. Suborbital plate is present, 

 but takes no part, apparently, in the ventral boundary of the 

 orbit, this being formed, as in the Anarthrodira, by the pre- 

 and postorbital elements. Jaws, ventral armoring, and endo- 

 skeleton not definitely known." 



One family, Chelonichthyida, thus defined: 



"Temnothoraci with orbits relatively small in size and 

 situated well forward in the head-shield. Occipital elements 

 produced antero-posteriorly, the external occipital forming the 

 posterior lateral angle of the head, no projection of the head 

 occurring in the region of the marginal plate. Median occipital 

 trapezoidal. Centrals take part in the median boundary of the 

 orbits, and embrace the pineal plate. Median dorsal with 

 poorly developed keel and terminal process." 



Heterosteus asmussi (perhaps to be called Ichthyosauroides 

 spinosus] is a gigantic species from the Lower Devonian of 

 Livonia. 



Allied to this species is Homostius milleri from Scotland, 

 celebrated as the "Asterolepis of Stromness" in Hugh Miller's 

 "Footsteps of the Creator." Another notable species is Homos- 

 tius formosissimus from the Lower Devonian of Russia. 



Arthrothoraci. --" Arthrodires whose dorsal shield articu- 

 lates with the head-roof by a conspicuous and movable peg- 

 and-socket joint, and leaves a definite interval (unprotected?) 

 between the two armorings. Orbits marginal, bounded in- 

 feriorly not by the suborbital element. In the head-shield the 

 postero-lateral angles formed by the marginal plate (Phlyctcs- 

 naspisf), the occipital border concave. A dorsal fin is present, 

 supported by endoskeletal elements." Five families, the most 

 important being the Coccosteida, thus defined: 



