134 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



American Species — Cont. 



R. mehillei Tansey 1922 



R. mesoflatys King 193 1 



R. mesoflatys baylorensis King 1931 



R. minima Savage 1 92 1 



R. missouriensis (Sv/iWow) 1860 



R. musculosa {Hi]l) 1857 



R.net'adensis (Meek) 1877 



R. nezvsomensis Yoente 1909 



R.oblata (Hall) 185 7 



R. occasus (Hall) I860 



R. oweni Hall and Clarke 1 892 



R. fecosi (Marcou) 1858 



R.feloris (Hall) 1863 



R. fenelofe (Hall) 1 860 



R. pennsylvanica (Simpson) 1889 



R. ferminuta Girty 1926 



R. freoblata Weller 1903 



R. saffordi Foerste 1903 



R.semele (Hall) 1863 



R. suborbicularis (HzW) 1858 



R. tenuicostata Wellei 1914 



R. tenuilineata Savage 1913 



R.thiemi (White) 1860 



R. transversa King 1 93 1 



R. vanuxemi {H&W) 1857 



R.vanuxemi fulchella (Herrick) 1888 



R. variabilis Grabau 1913 



Foreign Species 



Rhifidomdla altaica Obrut. 1926 



R. grandis Tolmachev 1912 



^. ,4ar//«' (Rathbun) 1879 



R. hybrida {^oviexhy) 1839 



R. imitatrix Diener 



R. inca (D'Orbigny) 1 842 



R. michelini (VExeiWe) 1835 



R. fenniana {Deihy) 1874 



R. -poly gramma (Sowerby) 1839 



R. foly gramma fentlandica (Davidson) 1 868 



R. reversa (McCoy) 



R. subcordiformis Kayser 



Distinguishing characters. — Rhiftdomella is 

 characterized externally by its nearly circular, or, in 

 later species, subtrigonal outline; the unequal con- 

 vexity of the valves, of which the dorsal is almost 

 always the more ventricose, and the ventral one usually 

 either markedly concave or showing a tendency in that 

 direction. The hinge-line is always very narrow. 

 Internally the ventral musculature is very characteris- 

 tic and the teeth are different from those in nearly all 

 other genera of the orthids. The dorsal valve, with 

 its arched umbo, has a ponderous cardinal process with 

 a very short shaft but an expanded myophore. The 

 cardinalia are distinctive and consist of widely divergent 

 brachiophores supported by inconspicuous deposits of 

 adventitious shell much as in Heterorthis. 



Discussion. — ^The external variations of the shell 

 are seen in outline and profile. Narrowing of the 



hinge is frequently accompanied by a lengthening of 

 the beak, producing thereby a subtrigonal outline. 

 This outline is characteristic of most of the Pennsyl- 

 vanian and Permian Rhipidomellas, but the Devonian 

 shells, with few exceptions, are mostly quite orbicular. 

 A fold and sulcus are rarely developed in true Rhtfi- 

 domella, and when present are inconspicuous. 



Internally there is a considerable variation in the ven- 

 tral musculature but the essentials of the muscle pat- 

 tern are uniform throughout the group. The most 

 important character is the complete enclosure of the 

 adductor scars by large semiflabellate diductor and 

 adjuster impressions. The adductor scars are always 

 situated at the posterior of the shell a short distance 

 forward of the pedicle callist. The size and shape of 

 the adjustor-diductor scar is very variable, "from one- 

 third to five-sixths the length of the valve." It varies 

 from circular to pentagonal and trigonal in outline. 

 The most extravagant development of muscles is in 

 R. musculosa in which nearly the whole floor of the 

 valve is covered. It is very difficult to differentiate 

 the adjusters from the diductors, but when this has 

 been possible the adjuster scars are seen to be rather 

 large. A median ridge usually divides the diductors 

 medianly and a small extension of it passes posteriorly 

 through the center of the adductor scar. This ridge 

 may be sharp and septum-like or it may be in the form 

 of a flat platform. It is not to be confused with the 

 euseptoid of Schizophoria which is a sharp septum bear- 

 ing the adductor scars. 



The dorsal valve shows variation in the strength of 

 the median ridge, which extends forward from the 

 shaft of the cardinal process and divides the adductors 

 medianly. Pallial marks are not commonly seen in 

 Rhipidomella, but those of R. emarginata show two 

 trunks, slightly divergent, extending forward from 

 the antero-median ends of the anterior adductors. A 

 lateral trunk extends from the ridges, dividing the ad- 

 ductors horizontally. These are elements similar to 

 those observed in Isorthis. 



The brachiophores are quite distinctive, being mod- 

 erately long, rather thick, somewhat bladelike, and at 

 the distal extremity are thickened or bear a toothlike 

 process on their outer ventral surface. This denticle 

 evidently serves to lock the tooth in place in the socket. 

 The latter is wide, being merely the space between the 

 brachiophore and the walls of the valve. Low flaring 

 supporting plates of extra shell lend strength to the 

 brachiophores. Additional strength is obtained by the 

 deposition of adventitious shell on the inner surface of 

 the brachiophores. Abbreviated chilidial plates are to 

 be seen rarely. 



Elongate blade- or spinelike processes extending 

 from the distal extremity of the brachiophores are 

 regarded by Hall and Clarke as crura. These proc- 

 esses are similar to others not infrequently seen in other 

 orthid genera. 



In some of the strongly biconvex Pennsylvanian Rhip- 



