62 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Hebertella vulgaris Raymond and Orthis imferator 

 Billings are tentatively placed under M'tmella. Both 

 species have the divergent diductor scars, the narrow 

 median ridge, and the thin cardinal process characteris- 

 tic of that genus. However, the typical pallial mark- 

 ings were not observed. The ventral musculature of 

 both species differs considerably from that of Orthh 

 sinuata Hall. 



Many specimens of Mimella have been referred to 

 P'tonodema in the past. Their external form is clearly 

 that of this genus but the impunctate nature of the test, 

 the trilobate ventral musculature, and the cruralium 

 should serve to differentiate the two genera. 



Genus SCHIZOPHORELLA Reed 1917 

 PI. 12, figs. 1-4, 7, 9 



Reed, Trans. Roy. Soc. Edinburgh, vol. 51, pt. 4, 1917, 



p. 858, pi. 10, figs. 19-24. 

 Cooper, Jour. Pal., vol. 4, 1930, p. 377, pi. 36, figs. 10, 



12, 13, 15. 



Genoholotype. — Orthh jallax Salter 1846,Synop. 

 Sil. Foss. Ireland, Add., p. 72, pi. 5, fig. 3. 



Description. Exterior. — Semicircular to subellip- 

 tical; margins and cardinal angles rounded or ob- 

 tusely angular; unequally biconvex, with the dorsal 

 valve usually the more convex; hinge-line straight, 

 narrower than the greatest width of the shell. An- 

 terior commissure uniplicate, fold and sulcus developed 

 best in front of the middle of the shell. Ventral inter- 

 area longer than the dorsal, plane or slightly curved, 

 strongly apsacline; ventral umbo convex, beak in- 

 curved. Dorsal interarea slightly curved or plane, 

 faintly apsacline to faintly anacline; umbo strongly 

 convex, beak scarcely defined. Delthyrium open; 

 notothyrium closed by the cardinal process. Orna- 

 mentation multicostellate ; test fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong, with oblique crural fossettes. Dental plates 

 strong, continued as a ridge around the margins of the 

 diductor scars and nearly uniting in front. Umbonal 

 cavities deep. Muscle area subelliptical, occupying 

 more than one-third the length of the valve. Diductor 

 scars elongate, enclosing a lanceolate adductor field; 

 adductor scars hemi-elliptical, divided centrally by a 

 ridge which separates the front ends of the diductor 

 scars and is continued forward for a considerable dis- 

 tance toward the front margin. Adjuster scars small 

 and obscure. Umbo-lateral spaces occupied by strongly 

 marked radial ridges which indicate the position of the 

 ovarian bodies. 



Dorsal interior. — Notothyrial cavity moderately 

 deep; cardinalia confined; sockets defined by a ful- 

 cral plate ; brachiophores strong, curved slightly, with 

 their supporting plates extended to the floor of the 

 valve; cardinal process plectorthoid. Adductor scars 



unequal in size, the posterior pair being the larger, 

 separated by low curved ridges extending antero-later- 

 ally from the median ridge. Internal surface marked 

 by oblique elongate ovarian ridges. 



Geologic range. — Upper Ordovician (Drum- 

 mock group) of the British Isles. The only species 

 known to the writers is the genotype, O. jallax Salter. 

 Distinguishing characters. — This genus is most 

 easily distinguished by its pionodemoid outline and pro- 

 file, hebertelloid dorsal interior, and the ventral muscle 

 field which has a number of variations from the pattern 

 usually seen in Hebertella. 



Discussion. — Through the kindness of the British 

 Museum we have been able to study specimens of the 

 genotype and of S. mullochiensis (Davidson), a Silu- 

 rian species that Reed referred to this genus. From 

 our studies it is apparent that the two species show a 

 remarkable instance of heterochronous homoeomorphy, 

 since the Ordovician S. jallax has an impunctate shell 

 and the Silurian form a punctate one. The speci- 

 mens studied have the following catalogue numbers: 

 S. jallax, B 52118, B 52121-41; S. mullochiensis, 

 B 44590-96, B 44607-10, and B 44615-16. Fortu- 

 nately specimens B 52136 and B 52118 of S. jallax 

 and B 44607-10 and B 44590-96 of S. mullochiensis 

 retain some of the original shell substance adhering to 

 the internal molds. This happy circumstance led us 

 to see the punctate test and accordingly to remove 

 S. mullochiensis from association in this genus, referring 

 it to the new genus Mendacella Cooper proposed in 

 1930 and discussed later in this work. 



Since Schizofhorella now proves to have an im- 

 punctate shell, the name chosen by Reed is unfortunate, 

 inasmuch as it implies relationship with the genus 

 Schizophoria. The genus under discussion, however, 

 clearly belongs in the Plectorthida; and in close asso- 

 ciation with Hebertella. 



Hebertelloid characteristics are seen in Schizophor- 

 ella in the plan of the ventral muscle field and the 

 nature of the cardinalia. The diductor impressions 

 have the general form seen in Hebertella, being some- 

 what expanded in front, thickened on the outer mar- 

 gins, and enveloping tightly the adductor impressions. 

 The latter are implanted on a low ridge, forming 

 together a narrow elliptical or lanceolate scar. The 

 median ridge is usually extended forward as a low 

 elevation nearly to the front of the valve. The dorsal 

 musculature, cardinal process, brachiophores and their 

 supporting plates are exactly like those of Hebertella, 

 and the sockets are defined by concave socket plates. 

 Some of the specimens of Schizophorella show the 

 brachiophore supports separated at their junction with 

 the valve, but this is a perfectly normal condition in 

 young shells throughout the Plectorthidae. 



Reed's genus is close to the American Doleroides 

 but differs in certain important details. The British 

 genus also appears later on the geological horizon. 



