ORTHIS. 133 



Orthis Michelim, De Koninck. Desc. des Animaux Fossiles du Terrain Carb. de la 



Belgique, pi. xiii, figs. 8 and W- d , 1843. 



— divaricata. M'Coy. Synopsis of the Carb. Fossils of Ireland, pi. xx, fig. 17, 



1844. 



CIRCULARIS. lb., fig. 19. 



— Michelim. De Verneuil el De Keyserling. Geol. of Russia, vol. ii, pi. xii, 



fig. 7, 1845. 



— — Semenow. Ueber des Fossilien des Schlesischen Kohlenbalkes, 



tab. iii, fig. 11, 1854. 



— — M'Coy. British Palaeozoic Fossils, tab. iii, fig. 11, 1854. 



— Michelini var., Burlington ensis, Hall. Iowa Report, pi. xii, fig. 4, 1858. 



— — Dav. A Monograph of the Carb. Brach. of Scotland, pi. i, figs. 



7—10, 1860. 



Spec. Char. Shell depressed, almost circular or subtrigonal, as wide or wider than 

 long, with greater width near the frontal margin, this last assuming a gentle outward or 

 slightly inward curve ; hinge-line much attenuated, not above one third the width of the 

 shell, sometimes less ; ventral area small, but rather larger than that of the opposite valve, 

 and divided by a fissure which is almost entirely filled up by the cardinal process of the 

 dorsal valve. Ventral valve either moderately convex, with a slight depression commencing 

 about the middle and extending to the front, or flattened throughout with the exception of 

 the beak, which is small, slightly incurved, and prominent. Dorsal valve, moderately and 

 uniformly convex, or gently depressed towards the front. Surface of both valves closely 

 covered with thread-like, radiating, rounded striae, which increase in number by means of 

 numerous interstriations and bifurcations, while from all the little ribs hair-like hollow spines 

 project and become more closely packed towards the margins. Intimate shell structure 

 perforated by small canals, of which the external orifices, under the shape of punctures, cover 

 the entire surface of the valves. 



In the interior of the dorsal valve, the cardinal process is situated between two pro- 

 jecting laminae (which perhaps afforded attachment to the spiral arms) while under this 

 shelly process a narrow median longitudinal ridge separates the quadruple impressions of 

 the adductor or occlusor muscle ; these last producing on either side two oval-shaped 

 depressions, one placed above the other, and separated by lateral elevations branching 

 from the central ridge. 



In the interior of the ventral valve, the dental plates extend to some distance along the 

 bottom of the shell, and between these are situated the muscular impressions. At the 

 base of the fissure, and between the dental plates, may be observed a horizontally striated 

 impression, which may perhaps be due to the pedicle muscle ; under this a smaller median 

 oval depression was produced by the occlusor, while the larger elongated depressions on 

 each side of this are apparently formed of two parts, the anterior or one close to the 

 occlusor being the cardinal or divaricator, while the other, the posterior or lateral, which 

 is parallel, may belong to the ventral adjustor ? Ovarian and vascular impressions are 

 also visible. 



