GENERA OF THE SUBORDER ORTHOIDEA 



99 



notothyrial cavity and cardinalia which was not noticed 

 in the genus under discussion. 



Midticostclla is believed to represent a side branch 

 of the D'ttwrth'is stock which flourished in Chazy time. 



Genus AUSTINELLA Focrste 1909 



PI. 9, figs. 12, 15-19 



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

 224. 



Genoholotvpe. — Orthis kankakcmis McChesney 

 1861, Desc. New Foss., p. 7 7 ; 1868, Trans. Chicago 

 Acad, Sci., vol. 1, p. 29, pi. 9, fig. 3. 



Description. Exterior. — Transversely subellipti- 

 cal to subscmicircular; hinge-line straight; cardinal 

 angles obtuse or nearly a right angle, rarely acute; 

 lateral profile subequally biconvex, anterior commissure 

 sulcate. Ventral palintrope broad and long, usually 

 gently curved, apsacline; beak slightly curved; del- 

 thyrium unmodified. Dorsal palintrope shorter than 

 the ventral, anacline, plane ; notothyrium unmodified. 

 Ornamentation multicostellate, some costellas hollow, 

 with concentric elevated growth-lines, strongest in the 

 interspaces; test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 large and strong, crural fossettes shallow, oblique; 

 dental plates thick, strong, grooved on the inside base 

 where the adjustor muscles are impressed; muscle area 

 quadrate or with anterior margin gently convex; mar- 

 gins thickened and elevated; adductor track straight, 

 expanding anteriorly, and occupying about one-fourth 

 the total width of the muscle area; diductor scars 

 straight and narrow; adjustor scars deeply impressed, 

 elongate; "vascular markings [pallial sinuses] leaving 

 the antero-lateral angles of the [muscle] scars, and 

 branching antero-laterally" (Foerste). 



Dorsal interior. — Notothyrial cavity shallow; car- 

 dinalia confined to the hinge region; brachiophores 

 short and thick, bluntly pointed, grooved on the inner 

 face, resembling those of Orthis or Dinorthis. Car- 

 dinal process of the Dinorthis type. A median ridge 

 extends from the notothyrial platform to the middle 

 of the valve. Adductor scars subequal, the anterior 

 ones subtriangular in outline; both pairs are divided 

 by a low ridge either at right angles or slightly oblique 

 to the median ridge. 



Geologic range. — Upper Ordovician and essen- 

 tially North American. 



American Species 



Orthis kankakensis McChesney 1868 

 H ebertelU icovUlei (M\\\cr) 1882 

 PUctorthis uhilfieUi (Winchell) 1881 



European Species 

 ? Orthis stracheyi Reed 1912 



Discussion. — Austinella is characterized by its 

 unequally biconvex profile, Dinorthis-Vikc ventral mus- 

 cle and palli.il markings and cardinalia. 



This genus was individualized first by Foerste, who 

 brought together species that had previously been in- 

 correctly assigned to Plectorthis and Dinorthis. The 

 several American species placed here form a compact 

 group but it is a difficult one to place satisfactorily in 

 any orthid family. The affinities place the genus near- 

 est to Dinorthis and Pltrsiomys but there are also pres- 

 ent Hehertella characteristics. The form of the valves 

 is not suggestive of Dinorthis but there is no reason to 

 suppose that a biconvex phase of that genus did not 

 exist. The ventral musculature in its quadrate form 

 and the prominent development of the adjustor scars 

 are ratiicr suggestive of Dinorthis. This resemblance, 

 however, is offset by the presence of a linear adductor 

 track expanding somewhat in its passage toward the 

 front and not included by the diductors, a feature that 

 is decidedly hebertelloid. Extending antero-laterally 

 from the front ends of each diductor scar is a short 

 pallial trunk. According to Foerste these trunks 

 branch antero-laterally but none of the specimens in 

 the Schuchert Collection supports this statement. The 

 presence of a main pallial trunk extending from the 

 anterior ends of the diductor scars is an almost univer- 

 sal feature of orthid shells, but it is significant that 

 these trunks in Austinella are divergent like those of 

 Dinorthis. They are in contrast with those in the 

 family Orthidse, which extend directly toward the 

 front of the shell, being nearly subparallel to the front 

 margin and then diverging abruptly laterally. On 

 the other hand, pallial trunks are rarely seen in 

 Hehertella but when present are short and not widely 

 divergent, probably because the ends of the diductor 

 scars are convergent. 



The cardinalia of Austinella suggest relationship 

 with Dinorthis rather than with Hehertella or Orthis. 

 The cardinal process has a crcnulated myophnre which 

 excludes it at once from the Orthidas. Winchell"" 

 notes the presence of a bilobed cardinal process in 

 A. whitfieldi, but this was not confirmed by the speci- 

 mens in the Schuchert Collection. It may have been 

 an old specimen in which lobation of the cardinal 

 process had taken place as in Dinorthis (Pla-siomys), 

 see pi. 9, fig. 20. The myophore of the cardinal 

 process is distinctly crenulated as shown by the one 

 dorsal interior at hand. Foerste"' mentions having 

 seen the interior of a dorsal valve in Dr. G. M. Austin's 

 collection, the cardinal process of which bears "a nar- 

 row groove down the middle posteriorly." Such a 

 groove is a common feature of Dinorthis (Pltrsiomys). 

 Excavation of the myophore into the shaft by the mus- 

 cles has been seen also in Dinorthis, so that the cardinal 



""Geol. N.it. Hist. Surv. Minnesota, 9th Ann. Rept., 

 for 1880, 1881, p. 116. 



" Bull. Denison Univ., vol. 17, 1912, p. 131. 



