DITHYROCARIS OVALIS. 141 



DithyhoCAEIS otalis, J. Armstrong, 1876. Catal. "W.-Scot. Fossils, p. 45. 



— — H. Woodward, 1877. Catal. Brit. Toss. Crust,, p. 73. 



— — Bigsby, 1878. Thesaur. Dev.-Carb., p. 249. 



— — J. Coutts, 1884-5. Trans. Geol. Soc. Glasgow, vol. viii, 



pp. 200 and 327. 



— — E., W., and J., 18S7. Kep. Brit. Assoc, for 1886 (1887), 



p. 64. 



— — Etheridge, 1888. Foss. Brit., vol i, Palreoz , p. 238. 



Size. — Length of single valves, probably 55 mm. ; breadth of single valves, 

 probably 18 mm. ; breadth of the two valves side by side, probably 38 mm. 



Specific Characters. — These are shown by the unique specimen here figured. 

 It consists of two valves crushed, flat, and open; imperfect at the edges. The 

 ventral border of each valve has left on the shale a strong impression of its 

 thickened margin, but no ornament is visible, except that a very small portion of 

 the postero-ventral margin of the left valve is preserved, with two or three 

 obscure prickles pointing backwards, just at the beginning of the large posterior 

 spine. 



The surface is smooth on the ventral region of each valve ; but the dorsal 

 regions are covered with numerous little triangular tubercles, with the apex 

 pointing backwards. A thin mesolateral ridge, very much depressed, evidently 

 formed of minute oblique rugse (as seen under the microscope, on the right 

 valve), exists on each valve, dividing the ventral from the dorsal region, and 

 reaching up to the cephalic region, where it is broken up by unequal pressure on 

 probably the gastric apparatus. On the left valve are faint indications of some 

 subsidiary parallel ridges. 



The junction of the dorsal edges of the two valves is very obscure ; they have 

 been squeezed together, and the right valve partly overlaps the other. Its 

 apparent edge and some longitudinal cracks simulate the relics of a middle dorsal 

 ridge, but are deceptive. 



The frontal notch seems to be neatly concave, but is somewhat obscured by 

 fracture. There are no indications of anterior spines. 



The posterior border, formed by the meeting of the curved ends of the two 

 valves, is much broken ; it seems to have had a deep mesial indentation. A 

 portion of the postero-ventral spine of the left valve may be recognised. 



From the Kirktonholm Cement- works, in black, non-calcareous shale above the 

 Calderwood Cement-stone of the Lower Limestone Group, East Kilbride. Mr. A- 

 Paton's Collection. 



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