304 G. G. Rohson — On Helininthochito,n. 



the nature of their sculpture. But wliile in II. grayicB both the 

 concentric and diagonal markings are prominent and clearly defined, 

 in the Bohemian form they are very faint and almost imperceptible, 

 and withal very different in character. Indeed, with regard to 

 S. grayicB, Mr. Cowper Reed's doubts (1907) as to its true zoological 

 position seem fully justified, and one would not be in any way 

 surprised if it were to be at a future date removed from the 

 Amphineura altogether. 



On the other hand, there is a significant point of agreement between 

 the Bohemian fossil and H. griffithii in the (posterior) excavation 

 above alluded to. In addition the valves are short and deep in both 

 species. On these grounds the new form may provisionally be 

 placed in the same genus as H. griffithii. It is distinguished from 

 that species by the possession of concentric and diagonal sculpture, 

 and the more acute angle which the two halves of each plate form 

 with each other. 



So far as I can discover, no Chiton has been recorded from the 

 Lower Ordovician before. Billings' problematic Chiton canadensis 

 (1865) is apparently the oldest-known member of the family up to 

 the present. This form was obtained from the Black River Limestone 

 of Llandeilian age (= Middle Ordovician). If H. mquivoca should 

 retain the classificatory position here assigned to it, it will rank as 

 the oldest-known representative of the Amphineura. 



The early Palaeozoic Chitons would appear to have been small 

 animals, possessed of an armature of thin valves, probably lacking 

 insertional laminse. In addition some of them were devoid of 

 sculpture on most of their valves, while others [canadensis, Billings, 

 the Silurian bohemica, Barrande, and the present one) were adorned 

 at least with a sculpture of fine striae. Differentiation into dorsal 

 and lateral areas was scarcely marked, and a well-developed carina 

 was probably present. 



In conclusion, the author wishes to express to Dr. F. A. Bather, 

 F.R.S., and Mr. F. R. Cowper Reed his acknowledgment of their 

 valuable assistance. 



Sehninthochiton csquivoca, n.sp. 



Yalves minute, strongly cai'inate in the median line, (posteriorly) 

 markedly emarginate, lacking insertional laminae, subquadrate. 

 Sculptured with two or three diagonal grooves, and with numerous 

 fine concentric striae running parallel to the free edges. Lower Ordo- 

 vician, Dj7 (= Arenigian), of Bohemia. Holotype in the Geological 

 Department of the British ^Museum (G 22212), from Male Prilepy. 

 Para type (G 22213) from Sarka. 



Papees referred to. 

 J. Barrande, Sijst. Silur. de la Boheme, vol. iii, p. 175, 1867. 



E. Billings, Palaozoic Fossils of Canada, Ottawa, 1865, p. 394. 



A. T. DE KOCHEBRUNE, Ann. Sci. Geol., vol. xiv, pp. 1-74, pis. i-iii, 1883. 



F. E. Cowper Eeed, Geol. Mag., 1907, pp. 108-15. 



J. W. Salter, in Griffith & M'Coy, Silurian Fossils of Ireland, Addenda, 

 1846, p. 71. 



Quart. Jom-n. Geol. Soc, vol. iii, pp. 48-52, 1847. 



H. Woodward, Geol. Mag., 1885, pp. 352-8. 



