E. Billings — Structure of Oholella chromatica. 177 



from near the cardinal scars forward about two-thirds of the 

 length of the shell. These diverge from each other, more or 

 less, in their extension forward, and are usually curved but 

 sometimes nearly straight. They may be called laterals. 

 They are. in general, separated from each other about one-third 

 of the width of the shell. A little above the mid-length, and 

 between the two laterals, there is a pair of small scars arranged 

 transversely, with their inner extremities directed somewhat 

 forward. The space above these two sears, between the upper 

 portion of the laterals, is generally tumid from the thickening 

 of the shell. In one of the specimens there is a small pit in the 

 center of this space. 



The dorsal valve has a small area, or nearly flat hinge facet. 

 The minute beak is slightly incurved over the edge of the area. 

 Beneath the beak there is a small sub-angular ridge, on each 

 side of which there is a cardinal ? scar. The elongated scars, 

 which seem to correspond to the laterals of the ventral valve, 

 are here altogether in the upper half of the shell. They 

 diverge widely in their extension forward. They are in general 

 very slightly impressed, and would, most probably, escape the 

 observation of any one who did not expect to find scars where 

 they are situated. In the cavity of the valve there is a low 

 rounded median ridge, which extends from a point near the 

 hinge line forward a little below the mid-length of the valve. 

 About the middle of the shell there are two small scars. These 

 are usually striated longitudinally. The median ridge passes 

 between them. The area is coarsely striated. 



The above are the principal characters of this species, and 

 they are subject to some variations, one of which is particularly 

 worthy of notice. The two small cardinal scars of the dorsal 

 valve are sometimes elongated laterally. This is carried to 

 such an extent in another species {0. gemma) that they not 

 only extend the whole length of the hinge-line, but are curved 

 forward at their outer extremities and continued down into the 

 cavity of the valve. In such cases they present an appearance 

 similar to that of the groove beneath the hinge-line of the genus 

 Oholellina. In other sDCcies of this genus the lateral scars of 

 the dorsal valve are sometimes connected together by their 

 upper extremities. But this is not a constant character. In 

 different individuals, of the same species, these scars are either 

 connected or not The laterals are also sometimes connected 

 with the cardinals. 



The following are the original figures published in the Pale- 

 ozoic Fossils, p. 7, (1861) : 



a I Fig. 4, o, Ventral valve; 6, dorsal; c, interior of 



jGk ^%. ilpk Itek ventral valve, showing the muscular impression ; d, out- 



^^ ^m Ci\ W line on a side view, restored from detached valves. 



^ ^ ^^ " Natural size. 



