156 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



larger, situated a little outside the anterior pair. A 

 prominent pallia! sinus extends antero-laterally from 

 the anterior end of each adductor muscle impression. 

 Geologic range. — Upper Cambrian to Lower 

 Ordovician of North America. 



American Species 



Syntrofhina 3 n. spp. (Ulrich Coll.) 



Syntrofhia cam f belli Walcott 1908 



S.isis Walcott 1924 



S. nuniina Walcott 1905 



S. fdmata Cleland 1900 



S.ferilla Walcott 1924 



S. rotundata Walcott 1905 



? Triflesia ■primordialis Whitfield 1877 



Distinguishing characters. — Syntrophina may 

 be recognized by its external resemblance to Syntro- 

 fhia and by the presence of a ventral spondylium, but 

 differs in having two dorsal divergent brachiophore 

 supports and consequently no cruralium simplex. 



Discussion. — Interesting features of this genus are 

 the well defined muscle marks on the dorsal side of the 

 spondylium, the peculiar shelf at the posterior of the 

 notothyrial cavity, and the dorsal muscle and pallial 

 marks. Internal molds from Phillipsburg, Quebec, 

 preserving the spondylia, show on them a median ridge 

 toward the anterior, which widens and becomes some- 

 what more elevated, terminating at the back end of 

 the median septum. This anterior ridge or elevation 

 may represent the impression of a deeply sunk adductor 

 track. Behind the "adductor track," and surrounding 

 it partially, are the probable impressions of the diductor 

 muscles on the floor and sides of the spondylium (see 

 pi. 15, fig. 15). The correct identification of these 

 muscle marks is attended with some difficulty, but the 

 suggested arrangement would be in accord with the 

 mechanics of opening or closing a brachiopod shell and 

 would homologize with the same marks seen in valves 

 not provided with a spondylium. 



In the dorsal valve the brachiophores project for 

 some distance into the valve and are supported by stout 

 plates which show as subparallel or slightly divergent 

 slots on internal molds (see pi. 15, figs. 2, 17, 19, 30). 

 These plates form the lateral walls of a deep notothy- 

 rial cavity, which has at the back end a low shelf built 

 on the sides of the brachiophore supports and the pos- 

 terior slope of the notothyrial depression. In the mid- 

 line of the valve and the shelf at its back end there is 

 in some specimens a low median elevation, interpreted 

 as the cardinal process (pi. 15, figs. 30, 31). This 

 shelf and the cardinal process probably mark the seat 

 of attachment of the diductor muscles. 



The adductor muscle scars are thickened at their 

 front ends and give off broad pallial sinuses that ex- 

 tend toward the antero-lateral extremities of the valve. 

 The musculature of Syntrophina strongly resembles 

 that of Syntrophioides and Finkelnburgia. 



The genus Syntrophina has a long series of species 

 (mostly undescribed) continuous from the Upper 

 Cambrian into the Lower Ordovician. So far as 

 known there are no Syntrophia s. s. in the Cambrian 

 or Ozarkian. It is true that Walcott considered 

 S. rotundata as the Cambrian representative of Syn- 

 trophia, but this appears not to be correct, since the 

 type specimens in the United States National Museum 

 have the characters of Syntrophina. The reference of 

 the species to Syntrophia is due to Walcott's mistaking 

 a ventral valve for a dorsal, thereby giving the essential 

 structure of Syntrophia. Other specimens in the same 

 lot (Cat. No. 52493), however, have the features of 

 Syntrophina. 



Ulrich's new genus Syntrophinella, as yet unde- 

 scribed, is internally like Syntrophina but is multi- 

 costellate externally. Its genotype is S. typica, n. sp., 

 illustrated on our plate 15, figures 4, 5, 13. 



Genus CLARKELLA Walcott 1908 



Pi. 15, figs. 6-11 



Walcott, Smiths. Misc. Coll., vol. 53, 1908, p. 110; 

 Camb. Brach., 1912, p. 809, pi. 104, figs. 2-2d. 



Genoholotype. — Polytaechia ? montanensis Wal- 

 cott 1905, Proc. U. S. Nat. Mus., vol. 28, p. 295. 



Description. Exterior. — Like Syntrophia, with 

 a prominent fold and sulcus. Test fibrous, impunctate. 



Ventral interior. — There is a spondylium simplex 

 but in some species it is supported by two or more 

 lateral accessory septa. Two strong divergent pallial 

 sinuses extend antero-laterally from the umbonal 

 cavities. 



Dorsal interior. — Essentially like that of Syntro- 

 phina, but there are accessory lateral septa, at most 

 two in number, which are united with the descending 

 brachiophore supports. There are six pallial sinuses 

 radiating from the umbonal cavities and in front of 

 the muscle area. 



Geologic range. — Upper Cambrian (Ozarkian). 



American Species 



Polytaechia montanensis Walcott 1905 

 Syntrofhia nanus Walcott 1924 



Distinguishing characters. — The character 

 which gives Clarkella generic standing is the structure 

 of the cardinalia with its prominent accessory plates. 



Discussion. — In the original definition of the genus, 

 Walcott described spondylia in both valves, supported 

 by accessory septa. This structure can not be ques- 

 tioned in the ventral valve, since there is here a spon- 

 dylium simplex supported by a stout median septum 

 and two or more accessory septa. The latter are, how- 

 ever, not universally present, being lacking in S. nonus 



