GENERA OF THE SUBORDER ORTHOIDEA 



61 



different family. H. ht-llatrix Reed" has the cariii- 

 nalia characteristic of Ort/tis s. s. and Hcspcrortlm, and 

 H. Ltptvorthi must be referred to Ptychoplcurella. 

 Foerste has referred to Hchrrtella a dorsal valve identi- 

 fied by Salter as Orthis retrorsti, calling it H. llanHcilo- 

 ensis, but since only the exterior was seen by Foerste 

 its reference to Hebertella is very doubtful.'" It is 

 therefore remarkable, when one considers the wide 

 distribution and abundance of Hebertella in North 

 America, that there should be in Europe no representa- 

 tives of the O. s'muata type. 



Genus MIMELLA Cooper 1930 



PI. 12, figs. 5, 6, 8, 10, 14, 15, 20; t. fig. 6 



Cooper. Jour. Pal., vol. 4, 1930, pp. 375, 382, pi. 35, figs. 

 9-12, 15. 



Genoholotype. — Pionodema globosa Willard 

 1928, Bull. Mus. Comp. Zool., vol. 68, p. 274, pi. 2, 

 fig. 5. 



Description. Exterior. — Externally like Piono- 

 dema of the Schizophoriid.-E, transversely subelliptical; 

 margins rounded ; cardinal extremities obtuse; hinge- 

 line straight; lateral profile subequally biconvex, the 

 dorsal valve usually the more so; anterior commissure 

 uniplicate; a slight sulcus developed on the ventral 

 valve only at the front of the shell. Ventral inter- 

 area long, curved, strongly apsacline; umbo strongly 

 convex, beak curved, delthyrium open. Dorsal inter- 

 area shorter than the ventral, faintly apsacline, umbo 

 very convex. Ornamentation multicostellate, elevated 

 growth-lines undulating over the whole surface. Test 

 fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 small; fossettes shallow; dental plates prominent, 

 receding, continued around the lateral margins of the 

 diductor scars as a low ridge; umbonal cavities deep; 

 muscular area large, trilobate; diductor scars diver- 

 gent, narrow, linear, not noticeably expanded in front; 

 adductor track linear, expanding toward the front, 

 occupying a strongly elevated track between the diduc- 

 tor scars; adjustor scars obscure ; pedicle callist small, 

 slightly elevated. Pallial markings prominent, consist- 

 ing of a wide trunk extending antero-laterally from 

 the anterior ends of the diductor scars. This trunk 

 bifurcates and sends a subsidiary one postero-latenilly 

 and another antero-medianly. Both of these split re- 

 peatedly into minor distributaries. The antero-median 

 curving of the two trunks noted above produced a 

 heart- or shield-shaped area occupied by elongated 



" Left unplaced. 



'^ The junior author has since seen plastotypcs of Hebert- 

 ella llanJeiloensis in the U. S. National Museum and is con- 

 vinced that the specimens are dorsal valves belonging to 

 Dinorthis (Retrorsirostra) . 



papillx in front of the muscle field. Such papillx 

 occupy all of the spaces not covered by pallial marks. 



Dorsal interior. — Notothyrial cavity deep, brachio- 

 phores not separable from their supporting plates. The 

 latter converge toward each other to unite with a 

 median ridge, forming thereby a sessile cruralium. In 

 young adults there are marked cavities beneath the 

 brachiophnre supports but in old forms these are filled 

 with adventitious shell. The sockets are shallow and 

 arc defined by a fulcral plate. Cardinal process a thin 

 linear ridge. The median ridge is thin and subcarinate 

 and extends for about one-half the length of the shell. 

 Adductor scars subequal. 



Geologic range. — Middle Ordovician (Chazy) 

 of North America. 



Species 



Pionodema globosa Willard 1928 

 P. minuscula WxWiri 1928 

 Hebertella melonica Willard 1928 



? H. vulgaris Raymond 1906 

 ?//. »OT/><rrd/or (Billings) 18 59 



Distinguishing characters. — The various spe- 

 cies of Mimella show a convergence toward the ex- 

 ternal form of Pionodema as striking as the homoe- 

 omorphy of Doleroides and Pionodema.^^ The genus 

 is characterized chiefly by its remarkable pallial mark- 

 ings, trilobed ventral muscle field, and a dorsal interior 

 having the features of Hebertella. 



Discussion. — Mimella is closely related to Hebert- 

 ella in the structure of the dorsal valve. The chief 

 distinction between the two seems to be in the delicacy 

 of the parts rather than in their general arrangement, 

 and this delicacy appears to be due to the earlier appear- 

 ance of Mimella. The cruralium is usually deeper and 

 more U-shaped in Mimrlla and the cardinal process 

 and median ridge are thinner. The cardinal process, 

 however, is of the plectorthoid type. The median 

 ridge is a rather high but thin partition, while in 

 Hebertella it is low and wide. 



The diagnostic feature of Mimella, as said above, 

 is the arrangement of the ventral pallial markings, and 

 it is the only genus of the Plectorthid.-e that has them 

 well developed, a feature in common with many of the 

 forms of Dinorthis {Plwsiomys). They differ chiefly 

 in that the postero-lateral branch of the main trunk, 

 so well developed in Plirsiomys, is sent off nearly 

 directly laterally in Mimella. This is probably due to 

 the fact that the muscles are more confined to the back 

 part of the valve in Mimella. Furthermore, the sub- 

 sidiary rami of the tertiary branches are less numerous 

 than in Dinorthis (Plttsiomys). 



"See Cooper, Jour. Pal., vol. 4, no. 4, 1930, pp. 369- 

 382. 



