116 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Description. Exterior. — Shell rotund to sub- 

 semielliptical, strongly biconvex; hinge-line straight, 

 usually narrower than the total width of the shell; 

 cardinal extremities obtusely angular or rounded. An- 

 terior commissure rectimarginate; dorsal sulcus obso- 

 lete or only faintly impressed. Ventral interarea longer 

 than the dorsal, curved, apsacline ; delthyrium closed 

 by a deltidium which is usually imperforate in adults. 

 Dorsal interarea short, orthocline to faintly apsacline 

 or anacline ; chilidium prominent. Surface finely mul- 

 ticostellate ; test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep, spondy- 

 lium longer than wide, sessile posteriorly but supported 

 by a low septum in front, the septum being continued 

 forward nearly to the front margin. Adductor ( ? ) 

 track occupying the central depressed portion of the 

 spondylium, diductor ( ? ) tracks on each side at the 

 base and partially on the sides of the dental plates. 

 Interior lateral spaces covered by subreniform ovarian 

 markings. Subparallel pallial impressions bounding the 

 inner margins of the ovarian markings and separated 

 by the median septum. 



Dorsal interior. — Notothyrial cavity and sockets 

 shallow, brachiophores orthoid, short, blunt, and sup- 

 ported as in Gonamhonttes. Cardinal process a linear 

 ridge on the thick notothyrial platform. Median ridge 

 strong, extending to the middle of the valve. Anterior 

 adductor impressions elongate, separated from the 

 shorter posterior pair by a narrow ridge at right angles 

 to the median elevation. Lateral spaces occupied by 

 pallial and ovarian markings. 



Geologic range. — Lower and Middle Ordovician 

 of northwestern Europe. 



European Species (of Pander 1830 except 

 where otherwise stated) 



H emifronit es i^jualis 



H. dta 



H. brevis 



H. circularis (pi. 16, B, is same figure as H. maxima) 



H . ex f ansa 



H. lata 



H . latissima 



H. maxima 



H. obtitsa 



H. orbicularis 



H. feralta 



H . ferlata 



H . ■prominens 



H. rotunda 



H. sfherica 



H. transversa 



H. tumida 



Orthis hemifronites Von Buch 1 840 



O. radians Eichwald 



? H. globosa (pi. 16, B, fig. 6). This is the same figure as 

 H . elongata and both have the appearance of a 

 hycofhoria 



? H. tenuistriata WeWcr 1907 (China) 



Distinguishing characters. — Hall and Clarke 

 used the name Hemifronites for shells with tumid, 

 subequally convex valves, hinge-line narrower than 

 the greatest width, the greatest thickness of the ventral 

 valve not at the beak, the surface finely multicostellate, 

 and the deltidium non-perforate. 



Discussion. — Hemifronites differs from its nearest 

 relative Clitamhonites s. s. in several important char- 

 acteristics. It is usually subequally convex, the ventral 

 palintrope is never procline, and the deltidium is im- 

 perforate. This contrasts strongly with Clitamhonites 

 in which the ventral valve is usually subpyramidal, 

 and the deltidium perforate at least in early maturity. 

 The ornamentation of Hemifronites is also different 

 in being much finer and in lacking the strong concen- 

 tric lamellje or frills of Clitamhonites. 



Internally there is also considerable variation be- 

 tween Hemifronites and Clitamhonites. In the ven- 

 tral valve of the former, to judge from H. tumida, 

 the spondylium is sessile for most of its length but at 

 its front end it is elevated on a low septum or ridge 

 which extends nearly to the front margin. The walls 

 of the spondylium are slightly convex inward and 

 converge rapidly, forming a narrow floor at the base 

 of the structure. This type of spondylium contrasts 

 with that of Clitamhonites in which it is sessile only at 

 the posterior and is elevated on a rather high septum. 

 The front end of the spondylium of Clitamhonites is 

 free and the floor rather wide. 



One of the specimens in the Schuchert Collection 

 (Cat. No. 369; see pi. 8, fig. 10) shows markings on 

 the spondylium that can be interpreted only as the 

 muscle impressions. Homologizing these marks with 

 similarly situated scars in orthids, the writers believe 

 that the central linear track represents the position of 

 the adductor muscles. This track occupies most, but 

 not all, of the narrow floor of the spondylium. It is 

 bounded on each side by low ridges. The diductor 

 impressions are thought to be narrow linear tracks 

 located outside the ridges bounding the adductor track 

 at the line where the dental plates unite with the 

 septum. The impressions of these tracks are also visi- 

 ble for a short distance on the inside surface of the 

 central lamellae. They are marked oS by scarcely 

 perceptible ridges along the margin of the muscle track. 

 Adjustor muscle impressions were not definitely 

 determined. 



Ovarian markings of irregular elevated lines are 

 also well exhibited in the same specimen. These are 

 subreniform areas such as are so common in Orthis, 

 Dolerorthlsy Paurorthls, etc. The shallow grooves 

 separated by the median ridge and bounded by the 

 ovarian impressions are taken to represent the two 

 main trunks of the pallial marks, homologous to the 

 two sinuses extending from the anterior ends of the 

 diductors of Orthis, Bllllngsella, etc. It is therefore 

 evident from the above that the important internal 



