NATURAL SCIENCES OP PHILADELPHIA. 2(51 



geological horizon. It differs, however, in several important specific charac- 

 acters, being proportionally much more gibbous, shorter, and, in consequence 

 of its hinge line forming a wider angle with its umbonal axis, distinctly less 

 oblique. It also differs in having its anterior side much less prominent and 

 more broadly rounded below the beaks, which consequently have the appear- 

 ance of being almost terminal. Its beaks are likewise more compressed 

 antero-posteriorly, and its hinge line shorter. Our specimen does not show 

 the cardinal area very satisfactorily, though it is evidently moderately well 

 developed and shorter than in Mr. Hillings' si'eeies. 



Until the hinge and interior of this shell can be examined, it is scarcely 

 possible to determine very clearly its generic character, but on comparison 

 with Cucullcea angustata, Sowerby, the type of McCoy's genus Dolabra,* and 

 other more obliquely truncated species, such as C. unilateraUs, Sowerby, C. 

 amydalina, Phillips, as figured in Phillips' Palaeozoic Fossils, we can scarcely 

 doubt the propriety of referring it to the genus Dolabra. Some of these 

 species have much the form and general external appearance of the genus 

 Cucull<ea ; while Sowerby's figure of an internal cast of the so called C. angus- 

 tata (Geol. Trans. (2), vol. v. pi. 53, fig. 25), seem to indicate a very similar 

 hinge. They appear to want the prominent posterior muscular support and 

 the radiating costas or striae of the more modern species of true Cuculltea, of 

 which, however, they are evidently palaeozoic representatives. 



Locality and position. Cincinnati group, of Lower Silurian Series, at Ster- 

 ling, Illinois. 



Mackodon micron em a, M. & W. 



Shell rather small, very inequilateral, elongate-oblong, nearly twice and a- 

 half as long as high, rather distinctly convex in the anterior and central 

 regions, as well as along tbe oblique posterior umbonal slopes. Posterior 

 dorsal region compressed above tbe umbonal ridge. Cardinal margin straight, 

 nearly parallel to the base, and but little shorter than the valves. Ventral 

 margin long and straight, or but slightly sinuous in the middle, and rounding 

 up rather abruptly and nearly equally at the ends. Posterior extremity trun- 

 cated, with a slight forward inclination, sometimes faintly sinuous in outline. 

 Anterior side very short and rounded. Beaks rather depressed, but rising 

 moderately above the hinge and somewhat flattened on the outer side ; in- 

 curved, approximate, and placed near the anterior end. Surface ornamented 

 with radiating striae, which are oblique, coarse, and rather irregular on the 

 compressed posterior region, but become gradually less oblique, finer and 

 more regular anteriorly, so that on the middle and anterior portions of the 

 valves they are exceedingly minute, very regular, and only visible by the aid 

 of a go >d magnifier in a cross light. A few moderately distinct marks of 

 growth are also seen near the basal and posterior margins. (Hinge, area and 

 interior unknown.) 



Length, 0-05 inch ; height (at beaks), 28 inch ; convexity, 24 inch. 



This little shell has much the form and general appearance of Macrodon 

 carbonaria,=( Area carbonaria, Cox, Kentucky Geol. Report, pi. viii. fig. 8), 

 but may be readily distinguished, not only by its smaller size and less nearly 

 terminal beaks, but by the extremely minute size of its radiating striae on 

 the convex portions of its valves. 



Locality and position. St. Genevieve County, Missouri, in the Chester divi- 

 sion of the Subcarboniferous series, also in the same position, Randolph Co., 

 Illinois. 



* The g^nus Dolabra, as first pr >poed by Prof. McCoy, included along with the typical species, 

 such as Cucuttsea angustata and C. uniUderalis, Sowerby, C. amydalina, Phillips, &c, other forms 

 belonging to the subsequently established, genus Schizodus, King. After the separation of the lat- 

 ter group, however, the name Dolabra was of course left, tor the other genus. 



I860.] 



