56 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



The structure of the dorsal interior of Finkeln- 

 hurgia is quite unique among the early brachiopods in 

 the possession of supporting plates attached to the brach- 

 iophores and fulcral plates, forming the sockets as in 

 Plectorthh. The cardinal process when present is a 

 simple, linear ridge. It is impossible to say from the 

 material studied whether it bore a crenulated myophore 

 or not. The callosities beneath the adductor scars are 

 another reminder of syntrophiid structure. 



Walcott's figures of this genus (pi. 93, figs. 1, 2) 

 are unsatisfactory and verbal corrections are necessary. 

 Figures id and if, of F. osceola, are typical ventral 



Family PLECTORTHID.E Schuchert and 

 Cooper 1930 



The Plectorthidae are biconvex or convexo-concave 

 Orthacea derived out of the Finkelnburgiidae, having 

 a rather variable ventral musculature, but in the dorsal 

 valve a uniform structure of the cardinalia. The 

 brachiophores are supported by convergent plates unit- 

 ing with the floor of the valve near the cardinal 

 process. Sockets are defined by small concave fulcral 

 plates. Cardinal process simple, with a compressed 

 crenulated myophore (in the ancestral genus Finkeln- 



Platystrophiinae 



Table 3 

 -Orthostrophiina 



Cyclocceliinx 



■\ / 



Plectorthidae 



Skenidiidae 



Finkelnburgiidae 



T 



2 



Orusia 



and dorsal valves having the characteristic features of 

 this genus when the two valves are in association. 

 Figure le is probably not a Finkelnburgia since it has 

 a rather strong sulcus, not seen in all the other speci- 

 mens referred to this genus. Figures Ig and Ih 

 appear to belong to Eoorthis. 



Figures 2 and 2e of the genotype, F. finkelnburgi, 

 are taken by us as characteristic of the genus. Fig- 

 ure 2 shows strong pallial markings and a prominent 

 median ridge such as occurs in F. osceola. Figure 2e 

 of the dorsal interior shows traces of the musculature 

 and a cardinal process, and displays all the characteris- 

 tics exhibited by better preserved valves accompanying 

 ventral ones of this species. It is unfortunate that 

 F. osceola was not selected as the genotype since well 

 preserved material of this or a closely related form 

 occurs in the Ozarkian (Gasconade) of Missouri. 

 The above description and discussion have been drawn 

 chiefly from two silicified valves of Finkelnburgia, 

 n. sp., in the U. S. National Museum (see pi. 13, figs. 

 13, 16, 17, 19), F. finkelnburgi, and F. armanda. 



burgia the cardinal ridge when present is devoid of a 

 myophore) ; it is in the Plectorthidae that the myo- 

 phore is seen for the first time. So far as known, the 

 delthyrium and notothyrium are open. 



Geologic range. — Middle Ordovician to close of 

 Silurian. 



Divisible into the following subfamilies; 

 Plectorthinas Schuchert 

 Cyclocoeliinje Schuchert and Cooper 

 Platystrophiinas Schuchert 

 ? Orthostrophiinas Schuchert and Cooper 



The genetic relations of these various subfamilies 

 appear to be as shown in Table 3. 



Discussion. — The Plectorthid.-e is one of the most 

 closely knit families in the Orthacea, paralleling the 

 Schizophoriidas in part of its development. The ven- 

 tral musculature is rather variable but this may be in 

 large part due to the different external form of some 

 of the genera. In general, however, it may be said 

 that the diductor scars are usually conspicuous, not 



