ARTHRODIRA. 
Genus HETEROSTEUS, Asmuss. 
% 
[Das vollkomraenste Hautskelet der bisher bekannten Thierreihe 
(Inaug. Dissert. Dorpat, 1856), p. 7 ( Heterostius ).] 
Syn. Ichthyosauroides, S. Kutorga, Zweiter Beitr. Geogn. u. Palaont. 
Dorpat’s, 1837, p. 35 (inappropriate). 
Cheloiiichthys, L. Agassiz, Poiss. Foss. vol. i. 1844, p. xxxiii. 
(name only, in part). 
Asterolepis, L. Agassiz ( non Eichwald), Poiss. Foss. V. G. R. 
1845, p. 89 (in part). 
A genus of enormous size, closely allied to Homosteus , but known 
only from detached dermal bones. Head and trunk broad, the dorsal 
aspect flattened ; scutes- ornamented with large stellate tubercles, 
and those of the upper surface of the head also marked with deep 
‘sensory furrows. A single broad median dorsal shield upon the 
trunk, with an inner longitudinal keel, and more less acutely 
pointed posteriorly ; the anterior dorso-lateral plate on each side 
with a very large, forwardly-directed, antero-external process, and a 
prominent condyle for articulation with the external occipital. 
Heterosteus asmussi (Agassiz). 
1837. Tvionyx spinosus , S. Kutorga, Zweiter Beitr. Geogn. u. Palaont. 
Dorpat’s, p. 9, pi. i. 
1837. Humerus Trionychis, S. Kutorga, ibid. p. 17, pi. viii. fig. 1. 
(P) 1837. Trionyx miliar is, S. Kutorga, ibid. p. 16, pi. vii. fig. 4. 
1844. Chelonichthys asmusii, L. Agassiz, Poiss. Foss. vol. i. p. xxxiii. 
(name only). 
1845. Asterolepis asmusii , L. Agassiz, Poiss. Foss. V. G. R. pp. 92, 
146, pi. xxx. fig. 1, pi. xxx. a. fig. 11. 
1845. Asterolepis, L. Agassiz ( non Eichwald), ibid. p. 94, pi. xxxii. 
figs. 7, 11-13, 15-19. 
1856. Heterostius hueckii, H. Asmuss, Das vollkommenste Hautskelet 
der bisher bekannten Thierreihe, p. 28. [Median dorsal plate; 
University Museum, Dorpat.] 
1856. Heterostius gracilior , H. Asmuss, ibid. p. 28. [Ditto.] 
1856. Heterostius convexus, H. Asmuss, ibid. p. 28. [Ditto.] *0 
1856. Heterostius eurynotus, H. Asmuss, ibid. p. 28. [Ditto.] 
1856. Heterostius ingens , H. Asmuss, ibid. p. 29. [Median occipital; 
University Museum, Dorpat.] 
1856. Heterostius secundarius , H. Asmuss, ibid. p. 30. [Anterior 
dorso-lateral; University Museum, Dorpat.] 
1856. Heterostius initialis, H. Asmuss, ibid. p. 31. [Ditto.] 
1857. Heterostius, O.H. Pander, Placoderm. devon. Syst. p. 82, pi. viii. 
fig. 1. 
