GENERA OF THE SUBORDER ORTHOIDEA 



129 



closely adjacent to it, a pallial trunk runs forward 

 nearly to the anterior margin. 



Dorsal interior. — Cardinalia confined; brachio- 

 phores large, divergent, bearing long blunt points given 

 off from the inside antero-ventral part; sockets very 

 deeply defined on the inner side by the sloping crenu- 

 lated outer face of the crural base and a small concave 

 fulcral plate which unites the brachiophore to the 

 floor of the valve. Denticle large; muscle area elon- 

 gate-oval in outline; anterior adductors the smaller; 

 peripheral ridges low; cardinal process small, bilobed 

 or trilobed, shaft produced forward to the line of divi- 

 sion between the diductors, where it is cleft. 



Geologic range. — Silurian. 



American Species 



Dalmanella concavoconi'exa Twenhofel 1928 

 D. ip-ingfelJemis Foerste 1917 

 D. wildroneniii Foerste 1917 

 Parmorthis crassicostata, n. sp. 



? Orihis media Shaler 1865 



European Species 



On his basalis Dalman 1828 

 O. eUgantulj Dalman 1828 

 O. visbyensis Lindstrom 1860 



Distinguishing characters. — This genus is dis- 

 tinguished externally by its plano-convex contour, 

 strongly inflated ventral valve, and peculiar ornamen- 

 tation. Internally, the stout brachiophores with their 

 blunt processes, oval muscle area, unusual articulating 

 apparatus with its crenulated teeth and sockets, and 

 ventral muscles and pallial sinuses all combine to indi- 

 vidualize the genus. From Dalmanella s. s., Parmor- 

 this is differentiated by its more inflated ventral valve, 

 different ornamentation, differently shaped ventral 

 muscle area, divergence of the brachiophore plates at 

 their junction with the floor of the valve, and possession 

 of a concave fulcral plate. It differs from Isorthis in 

 not possessing a complicated pallial system and in cer- 

 tain details of the cardinalia. 



Discussion. — There is a feature of the ornamenta- 

 tion of these shells which is characteristic. In the 

 sulcus of the dorsal valve there is usually a concentra- 

 tion of costellae due to the accelerated appearance of 

 secondary and tertiary ribs. This frequently produces 

 a nearly smooth or nearly plane area in the front of 

 the sulcus that is ordinarily elevated as a slight fold in 

 its center. There is a corresponding smooth area in 

 the median portion of the ventral valve which is com- 

 monly depressed slightly below the general level of 

 the shell. Thb peculiarity occurs in nearly all of the 

 species studied. 



The articulation of Parmorthis is unusual for the 

 strength of its parts and their specialization. It is the 

 only genus known among the orthids (except Trofid- 

 oleptus, now placed in the Dalmanellacea) that possesses 



ci cnulated teeth and sockets. The details of the articu- 

 lation are described on page 25. 



Noteworthy also in this genus is the occurrence of a 

 small deposit of secondary shell in the apex of the del- 

 thyrium. This has nothing whatever to do with a 

 deltidium, being merely an apical callus such .is occurs 

 commonly in some spiriferid genera. In the dorsal 

 valve the brachiophores not uncommonly bear elon- 

 gate, bluntly pointed processes on their inner dorsal 

 and distal extremities. If these and similar processes 

 occurring in the same position in other orthid genera 

 could be homologized with the crura of the rhyncho- 

 nellids, it would be possible to prove that the brachio- 

 phores in reality are the same as the crural bases. 



"Dalmanella" visbyensis (Lindstrom) shows an in- 

 teresting variation from the usual type of Parmorthis. 

 In this form the ventral beak is more strongly arched 

 than is usual, actually overhanging the dorsal interarea. 

 Along with this variation goes a concave dorsal valve. 

 Within the dorsal valve the myophore of the cardinal 

 process is directed backward (posteriorly and dorsally) 

 so far as to be visible from the dorsal side. Seen from 

 the inside, one perceives that the inner margins of the 

 shaft and the brachiophore plates are grown together. 



Orthis elegantula Dalman differs importantly from 

 our Dalmanella and from Resserella. In Dalmanella 

 the ventral muscle field is lobate and the pallial mark- 

 ings widely divergent, whereas in Parmorthis the ven- 

 tral pallial markings are closely appressed and sub- 

 parallel, and the muscle area is never lobate and is 

 confined to the delthyrial cavity. In the dorsal valve 

 important differences also occur. The adductor field 

 of Parmorthis is elongate, and in our Dalmanella it is 

 subcircular. Further, the sockets of Parmorthis are 

 defined by concave fulcral plates, which are lacking in 

 Dalmanella. These differences we regard as of sufl^- 

 cient importance to separate the two genera and place 

 them in different families. 



Parmorthis crassicostata, n. sp. 

 PI. 21, figs. 4, 5 



Shell rather small, subcircular, of the type of P. ele- 

 gantula but having rather coarse ribs, there being about 

 34 to the valve. Measurements of the holotype: 

 length, 9 mm.; width, 9 mm. 



Holotype, Cat. No. 913, Schuchert Collection, Yale 

 University. Silurian (Niagaran), Martin's Mills, 

 western Tennessee. 



Genus FASCICOSTELLA Schuchert and 

 Cooper 1931 



(Lat. jasc'uy bundle; costella, a little rib) 



PI. 22, figs. 6, 7, 10-12, 15, 16, 31 



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

 p. 246. 



