GENERA OF THE SUBORDER ORTHOIDEA 



87 



dclthyrium or notothyrium is the aberrant, probably 

 primitive, Nicolflln. The ensemble of characters ex- 

 hibited by each of these two groups of shells therefore 

 makes them distinct, although their surficial resem- 

 blances would lead the more casual observer to classify 

 them under the same name. 



Hall and Clarke homologized the "apical plate" of 

 Orthis tricrtiaria with that seen in Spirifer. They 

 say:"" 



In this group of orthids [the authors here doubtless have 

 reference to the American shells], more frequently than 

 elsewhere, we find a character rarely developed in any stage 

 of growth, viz: the existence of a transverse apical plate in 

 the delthyrium of the pedicle-valve. This is probably ho- 

 mologous to the apicil plate of the Spirifers, but is wholly 

 distinct in origin from the covering of the delthyrium. 



As we see this matter, H.ill and Clarke are in error 

 in homologizing the apical plate of Spirifer with that 

 of Orthis iricenaria. In the latter the apical plate 

 differs from a true deltidium in its lack of convexity, 

 which is the usual condition of the deltidium; it is a 

 flattened arch in the delthyrium with its piers built 

 against the sides of the dental plates, and it is quite 

 likely that the deltidium never attains any great degree 

 of convexity because the palintrope overhangs the mar- 

 gins of the delthyrium for some distance. In O. tri- 

 cenaria this flattened arch is formed in precisely the 

 same position as that of O. latirt'tttina Billings, but here 

 it is convex and has never been regarded as anything 

 but a true deltidium, a structure that never would be 

 homologized with the plate in "Spirifer." In fact. 

 Hall and Clarke regarded the presence of the convex 

 plate in O. laurentina as so significant that they placed 

 the species in the genus Billingselb, which has a con- 

 vex deltidium. Since the plate of O. tricenaria is 

 formed in ex.ictly the same way as that of O. laur- 

 entina, and the internal anatomy of the two species 

 is the same, the writers concluded that the "apical 

 plate" of the former is a true deltidium. On the other 

 hand, the "apical plate" of "Spirifer*' is not an arch 

 with its piers abutting against the sides of the dental 

 plates; on the contrary, it is, in many genera, a solid 

 callosity filling the apex and not infrequently never 

 rises above the level of the interarea. In Spirifer this 

 callosity is clearly for pedicle attachment, but the del- 

 tidium may also serve the same function though in the 

 reversed way. Even though the two plates may serve 

 the same function, they are, however, not homologous 

 since they originate in different ways. 



In America Hesperorthis appears first in the Middle 

 Ordovician (Chazy and Ottosee) and is seen last in 

 the Middle Silurian (Chicotte) of Anticosti. In 

 Europe the genus appears first in O. inostrantzefi 

 Wysogorsky of the later Middle Ordovician (Wier- 

 land group, Cii^, which is about Black River in age), 

 and ranges into the late Silurian (Wenlock). 



"Pal. N. Y., vol. 8, pt. l,p. 193. 



Genus SCHIZORAMMA Foerste 1912 



PI. 5, figs. 3,6, 11, 13, 14, 16; t. fig. 4 



Foerste, Bull. Sci. Lab. Denison Univ., vol. 14, 1909, p. 

 77, pi. 3, figs. 45A, B (as Schizonema = homonym) ; 

 vol. 17, 1912, p. 139. 



Genohoi.otype. — Hebertclla (Schizonema) fissi- 

 striata Foerste 1909. 



Description. Exterior. — Shell semicircular to 

 semielliptical, cardinal angles obtuse, hinge-line straight, 

 lateral profile plano-convex to unequally biconvex; 

 anterior commissure rectimarginate, sulcate, or faintly 

 uniplicate; ventral interarea long, strongly apsacline, 

 delthyrium open or partially closed by tlie overhanging 

 palintrope or a small deltidium; beak not incurved; 

 dorsal interarea very slightly developed, faintly or mod- 

 erately anacline, beak very small; ornamentation mul- 

 ticostellate to fascicostellate, with strong elevated 

 growth-lines or filx, especially in the interspaces. 



Ventral interior. — Delthyrial cavity shallow, teeth 

 strong, triangular, placed laterally from the delthyrial 

 margin; dental plates strong, thin, receding; palin- 

 trope overhanging the delthyrial cavity and simulating 

 deltidial plates; umbonal cavities deep; muscle area 

 small, obcordate, one-fourth the length of the shell in 

 mature forms; commonly marked by a thickening 

 extending from the base of the dental plates; adduc- 

 tor track narrow, linear, extending to the anterior 

 margin of the muscle area; diductor scars subtriangu- 

 lar, divergent; two pallial trunk sinuses extending 

 from the antero-median edges of the diductors nearly 

 to the anterior margin, and defining reniform ovarian 

 impressions as in Dolerorthis and Hesperorthis ; external 

 strije impressed on the margin of the shell, each bearing 

 a groove. 



Dorsal interior. — Notothyrial cavity shallow, cardi- 

 nalia confined; brachiophores short, orthoid, margin- 

 ing the notothyrium, somewhat spoon-shaped because 

 of adventitious shell under the ventral edge in front, 

 and bearing the shallow sockets; cardinal process 

 linear, expanded in front; in some specimens there is 

 an elevation on the notothyrial platform on either side 

 of the cardinal process (see pi. 5, fig. 14). A strong 

 broad median septum extends from the cardinalia to 

 the middle of the shell, dividing the muscle-scars; ad- 

 ductor impressions subequal, separated by a thin, ob- 

 scure ridge at right angles to the median septum. 

 Pallial markings obscure but evidently there are two 

 somewhat curved ones radiating from the front margin 

 of each anterior adductor. 



Geologic range. — Middle Silurian. 



American Species 



Orthoslrofhia (Sc/iizoramma) fssistriala (Foerste) 1909 



Hebertella (Schizonema) celsa Foerste 1909 



Orthis fasciata Hall 1852 



O . fissiflica RoemcT 1860 



O. nisii Hall and Whitfield 1872 



