DITHYROCARIS BELLI. 177 



PART IV. 



11. Dithyrocabis Belli, H. Woodward, 1871. Plate XVIII, figs. 8 a, b, c. 



Dithtkocaeis stbiatus, H. Woodward, 1871. Rep. Brit. Assoc, for 1870, 



Sections, p. 90. 

 Bellt, H. Woodward, 1871. Geol. Mag., vol. viii, p. 106, pi. iii, 

 fig. 5. 

 — 1872. Canadian Naturalist, vol. vi, pp. 18, 



19. 



— S. A. Miller, 1877. Americ. Palseoz. Foss., p. 217. 



— Bigshy, 1878. Thesaur. Dev.-Carb., p. 27. 



— Packard, 1883. Monogr. North-American Pbjll. Crust., 



p. 452. 



— — EtTieridge, Woodward, and Jones, 1887. Eep. Brit. Assoc. 



for 1886, p. 65. 



— — Hall and Clarke, 1888. Geol. Surv. New York, Palaeon- 



tology, vol. vii, p. 194. 

 Mesothtba Belli, & A. Miller, 1889. N. -American Geol. Palseont., pp. 545 



and 556. 

 DiTUTROCAErs Belli, A. W. Vogdes, 1893. Bibliogr. Palaeoz. Crust., p. 3S2. 



Size. — Length (imperfect) 83 mm.; breadth of the two valves, shifted one on 

 the other partially, 25 mm. ; breadth of single valve probably 13 mm. 



Specific Characters. — Two valves, both imperfect anteriorly. The right valve 

 shifted forwards and partially overlapping the other at the dorsal edge; its hinder 

 border obscure. The left valve has its posterior spine and postero-ventral region 

 fairly exposed, with its serrated margin (fig. 8 b). 



Each valve bears a slightly curved mesolateral ridge, marked with chevrons 

 pointing backwards (downwards in figs. 8 a, b). The two mesolaterals are brought 

 close together in front by the oblique shifting of the valves one on the other. 

 The surface of the valves is ornamented with numerous delicate riblets, parallel 

 with the mesolateral ridge and the ventral margin ; some die out without reaching 

 the posterior region. The interstices between the riblets are filled with a trans- 

 verse and oblique irregular reticulation (fig. 8 c). An outer or second lateral 

 ridge near the ventral margin is just visible, but rather more distinctly on the 

 right than on the left valve. 



No eye-spot is present, but a slight unevenness of the surface between the 

 incurved ridges near the posterior end seems to have been taken for it in the 



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