GENERA OF THE SUBORDER ORTHOIDEA 



79 



Discission. — The tongue-like process in the ven- 

 tral musculature of Glossorthis is its most characteristic 

 variation from Orthis s. s. Opik reports a spondylium 

 in the ventral valve, but in reality the structure is a 

 fine example of a pseudospondylium. As with other 

 brachiopod genera, the anterior part of the floor of the 

 delthyrial cavity may become so thickened by the depo- 

 sition of callus under the muscles as to form a muscle 

 platform. The callus also spreads upon the sides of the 

 dental plates as a wash, making the ensemble structure 

 appear as a spondylium. In an old example of this 

 genus kindly sent to Schuchert by Opik these phenom- 

 ena arc exhibited to perfection. The front part of 

 the pseudospondylium is composed of adventitious shell, 

 with an elevated marginal rim around the diductor 

 scars. Traced backward, these rims can be followed 

 just beyond the front margins of the dental plates, 

 where they die out. The dental plates extend outside 

 the adventitious material to the base of the valve. 

 The tongue-like feature is evident on young shells but 

 more rapid shell deposit under the adductor scars 

 emphasizes it. Such a pseudospondylium occurs also 

 in our genus Linoforella. 



Another feature of considerable importance well ex- 

 hibited in Glossorthis is "deltidial plates" (see pi. 4, 

 fig. 28). The teeth are set far outside of the delthyrial 

 margins and there is a line of growth visible on the 

 interarea, showing their progressive forward move- 

 ment. But on the inside end of the tooth there is 

 another suture line extending to the apex and marking 

 the line of junction of a marginal plate with the inner 

 wall of the delthyrium. It is these marginal plates 

 that limit the delthyrium and produce the shelves of the 

 palintrope which overhang the delthyrial cavity. Sim- 

 ilar structures are known in Ptychopleurella and 

 Uesferorthts. 



In the dorsal valve the most interesting anatomical 

 features of the genus center about the cardinal process. 

 It is thin and bladelike, thickened on its ventral edge. 

 The thinning is due, evidently, to attachment of mus- 

 cles on its sides. Pits on the sides of the notothyrium 

 and floor of the notothyrial platform suggest muscle 

 attachments in these places also, as in other members 

 of the Orthidae. 



Opik believed that Glossorthis showed relationships 

 with Platystrophia, but a study of the cardinalia and 

 brachiophores at once reveals its intimate relationship 

 with Orthis s. s. and its allies. 



Genus PAURORTHIS Schuchert and Cooper 



1931 



(Gr. fauroSy small) 



PI. 3, fjgs. 5-8, 10 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 243. 



Genoholotype. — Orthambonites farva Pander 

 1830, Geogn. Russlands, p. 83, pl. 26, fig. lOa-c. 



Description. Exterior. — Shell small, dalmanclli- 

 form, subcircular, cardinal extremities rounded; hinge- 

 line straight, shorter than the greatest width ; lateral 

 profile subequally biconvex, the ventral valve having 

 the greater convexity; lateral commissure straight or 

 ventrally flexed anteriorly; anterior commissure recti- 

 marginate in young shells but broadly sulcate in adult 

 forms; dorsal sulcus shallow, but pronounced. Ven- 

 tral interarea short, faintly apsacline to orthocline, 

 curved; ventral umbo swollen, beak strongly curved, 

 delthyrium open, beak not uncommonly resorbed to 

 allow the passage of the pedicle. Dorsal interarea 

 shorter than the ventral, faintly anacline, umbo gently 

 convex, sulcate; notothyrium open. Ornamentation 

 fascicostellate. Test fibrous, impunctate. Exterior 

 layer pierced by pits or exopuncts. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong, crural fossettes deep; dental plates strong in 

 youthful shells, but obsolete in adult specimens, extend- 

 ing vertically to the floor of the valve ; muscle area 

 deeply impressed, slightly elevated in front, longer than 

 wide, occupying about one-third the length and one- 

 fourth the width of the shell. Adductor and diductor 

 impressions not divisible, forming a wide track in the 

 center of the delthyrial cavity and occupying nearly its 

 whole width; adjustor (?) scars located on the sides 

 of the delthyrial cavity. In front of the muscle area a 

 broad or narrow median ridge extends forward nearly 

 to the anterior margin, dividing the ovarian impres- 

 sions, which are reniform in outline and occupy the 

 lateral spaces of the interior. Anterior margin 

 crenulate. 



Dorsal interior. — Notothyrial cavity shallow; brach- 

 iophores of the Orthis type, being divergent plates 

 placed obliquely to the roof of the valve; extremities 

 subacute; sockets deep, formed by the broad sloping 

 outer face of the crural base and the roof of the valve. 

 Brachiophores supported only by a thickening of the 

 notothyrial platform. Cardinal process a simple, low, 

 linear ridge, or absent; a rounded median septum ex- 

 tending forward from the notothyrial platform to a 

 point slightly beyond one-half the length of the valve. 

 Muscle field small, anterior adductor scars larger than 

 the posterior pair; diductor scars visible on each side 

 of the cardinal process on the notothyrial platform. 

 Ovarian impressions forming two subreniform lobes 

 on the lateral spaces, the anterior ends of the lobes 

 extending only slightly anterior to the front margin 

 of the muscle area. 



Geologic range. — Lower and Middle Ordovi- 

 cian (Bifl-Biiie) of Europe, and North America. 



European Species 



Orthambonites farva Pander 1830 

 O. trigona Pander 1830 

 Orthis farvula Lamansky 1905 

 Pro Justus Utissimus Vinder 1830 

 P. minimus Pander 1830 

 P. minulus Tinder 1830 



