70 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



plates convergent and uniting under the cardinal proc- 

 ess. The latter is a thick ridge with an unexpanded 

 myophore as in Plectorthis. The sockets are defined 

 by fulcral plates. 



One interesting variation between mature Platystro- 

 fhla and Mcewanella is found in the different degree 

 in which the two genera thicken the shell. In nearly 

 all Platystrophias, even the small Russian species, this 

 thickening may go on to a prodigious degree. But in 

 the largest dorsal valve of Mcewanella studied, which 

 is 35 mm. wide, there is practically no deposition of 

 adventitious shell, the delicate little fulcral plate being 

 readily visible. This variation of growth habit em- 

 phasizes the generic distinction between the two groups. 



It is obvious from the decided similarity in both the 

 internal and external characters of Mcewanella and 

 Platystrofhia that the origin of the former should be 

 sought in one of the groups of Platystrofhia. Accord- 

 ing to McEwan's view (p. 429) "the greater strength 

 of the median plications suggests that it [M. lineolata] 

 belongs to the Triplicate Type." In her table 'on p. 

 403 she places M. lineolata (her P. jernvalensis) in 

 the Ponderosa subgroup of the tricostate division. We 

 have studied McEwan's material and are of the opinion 

 that this species belongs in the unicostate division. A 

 ventral valve (a specimen figured on pi. 50, fig. 1) 

 has a prominent median rib in the sulcus. A dorsal 

 valve (pi. 50, fig. 3) shows two costs on the fold 

 at the rear third of the shell but these are broken into 

 costella; at the front which occupy the whole sulcus. 



The other species, M. raymondi, indicates the group 

 affinities much more closely than M. lineolata. Here 

 the dorsal valve as figured by Foerste^^ has two costas 

 in the fold, while a ventral valve collected by Ulrich 

 from the top of the Kimmswick at Cape Girardeau, 

 Missouri (pi. 12, fig. 17), has a prominent median 

 costa in the sulcus. It would appear therefore that 

 Mcewanella came out of the unicostate division rather 

 than the tricostate one. 



The Kimmswick limestone, from which M. ray- 

 mondi is said to have come, is usually regarded as of 

 Trenton age, and if so, Mcewanella arose near the 

 time of origin of the Platystrophias. On the other 

 hand, the upper Kimmswick (the horizon carrying 

 Mcewanella) may be of Richmond age. In any event, 

 the two species of Mcewanella are of one genetic line, 

 showing origin in the unicostate group of Platy- 

 strofhia. 



Subfamily ORTHOSTROPHIIN^ Schuchert and 

 Cooper 1931 



Convexo-concave to unequally biconvex Plector- 

 thidas, having confined ventral musculature and dorsal 

 muscles elevated on a callosity. Both valves have 



" Bull. Denison Univ., vol. 19, 1920, p. 198, pi. 23, 

 fig. 1. 



complicated pallial markings, which resemble those of 

 Mimella in the ventral valve, and it is on the basis of 

 this feature that Orthostrofhia, the only genus of this 

 subfamily, is placed in association with the members 

 of the Plectorthidas. 



Geologic range. — Silurian into Lower Devonian. 



Genus ORTHOSTROFHIA Hall 1883 



PI. 6, figs. 22-25, 27, 28, 31; t. fig. 7 



Hall, 2d Ann. Rept. N. Y. State GeoL, 1883, pl. 36, figs, 



32-34. 

 Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 1892, p. 199, 



pl. SA, figs. 24-27, pl. 6, figs. 32-34. 



Genoholotype. — Orthis strofhomenoides Hall 

 1857, 10th Rept. N. Y. State Cab., p. 46. 



Description. Exterior. — Shell sub-strophomenoid, 

 hinge-line straight and wide, cardinal extremities 

 rounded or obtuse, slightly deflected; lateral profile 

 convexo-concave or subequally biconvex; anterior 

 commissure uniplicate or sulcate; fold and sulcus fre- 

 quently the reverse of the normal. Beaks incurved, 

 commonly approximate. Ventral area the longer, ap- 

 sachne; dorsal area shorter, orthocline or apsacline; 

 delthyria and notothyria open; ornamentation multi- 

 costellate ; costellje and interspaces crossed by elevated 

 concentric lines of growth. 



Ventral interior. — Delthyrial cavity shallow or 

 deep; teeth strong, dental plates thick, obscured by 

 adventitious deposit; umbonal cavities almost com- 

 pletely obliterated in old shells; muscle area confined, 

 adductor track linear, expanding in front; diductor 

 tracks small, subtriangular ; pallial markings promi- 

 nent, two main trunks extending forward from the 

 ends of the diductors and two other trunks extending 

 anteriorly but soon bifurcating, each of the branches 

 sending out ramifications. Between the two lateral 

 trunks are two minor sinuses. 



Dorsal interior. — Notothyrial cavity shallow, car- 

 dinalia thickened, brachiophores thick, blunt, sockets 

 shallow; cardinal process linear, expanded anteriorly; 

 there is not uncommonly a small ridge on either side of 

 the cardinal process, as in Schizoramma. Muscle area 

 confined, deeply impressed, the margins of the field 

 being thickened and elevated. Adductor scars sub- 

 equal in size, the anterior pair usually the more clearly 

 impressed and the more elongate. Pallial markings 

 strongly impressed; two large divergent trunks ex- 

 tending anteriorly from the antero-lateral ends of the 

 anterior adductors, and each sending off three or four 

 branches which bifurcate near the margin, each bifur- 

 cation in turn sending off ramifying branches. On 

 each side of and adjacent to the muscle area in the 

 umbo-lateral spaces is a more or less circular ovarian 

 area marked by raised dendritic lines. 



Geologic range. — Silurian into Lower Ordovi- 

 cian of eastern North America. 



