GENERA OF THE SUBORDER ORTHOIDEA 



45 



in the genotype and other species. Micro-structure of 

 the shell not definitely known, probably fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity obsolete; teeth 

 very rudimentary; dental plates absent, ventral muscle- 

 scars not visible. 



Dorsal interior. — Notothyrial cavity very shallow; 

 notothyrial callosity confined, without a cardinal proc- 

 ess, and not produced forward as an axial ridge or 

 thickening. Brachiophores bladelike thickenings along 

 margins of notothyrium. Adductor scars appearing 

 as two divergent, elongate impressions anterior to the 

 notothyrial callosity. 



Geological range. — Late Lower and early Mid- 

 dle Cambrian. 



American Species' 



Billingsella Jberla (Walcott) 1888 

 B. festinata (Billings) 1861 

 B. transversa (Walcott) 1886 

 Nisusia burgessensis Walcott 1924 



? BUlingsella bivia Walcott 1912 



? B. orientalis (Whitfield) 1884 



? Nisusia (Jamesella) amii Walcott 1905 



? N. (J.) erecta Walcott 1905 



?A'. (y.) /o»»" Walcott 1908 



? iV. (y.) nautes (Walcott) 1905 



? Af. (y.) sfencei (Walcott) 1905 



? iV. (y.) utahensis Walcott 1905 



Distinguishing characters. — The genus Nisusia 

 is characterized especially by the biconvex profile of the 

 valves, the well developed deltidium with apical per- 

 forations (in the genot)'pe), a chilidium, a lack of 

 dental plates and of a cardinal process, and very primi- 

 tive cardinalia. Walcott (1912) regarded it as the 

 most primitive of known Protremata. 



Discussion. — Since Nisusia is the oldest known 

 articulate brachiopod of the Cambrian, it has great 

 taxonomic importance. Accordingly the species of the 

 genus have, perhaps, the most primitive structure. The 

 valves are biconvex, the dorsal having as a rule greater 

 convexity. The ventral valve has a prominent sulcus 

 and the surface of the shell is marked by costelL-e which 

 increase both by intercalation and by bifurcation. In 

 the genot)'pe and some of the other species the crests 

 of the costellx are provided with prominent spines. 

 This spinose surface Walcott considered as the dis- 



' Generic designations used in these lists are taken from 

 the following sources: 



For American species prior to 1897, Schuchert, Bull. 87, 

 U. S. Geol. Surv., 1897. 



For Cambrian species from 1897 to 1912, Walcott, 

 Mon. 51, U. S. Geol. Surv., 1912. 



For American Ordovician and Silurian species from 

 1897 to 1915, Bassler, Bull. 92, U. S. Nat. Mus., 1915. 



Other species are, in the main, cited under their original 

 generic designation. 



tinguishing feature between Nisusia and Jamesella, 

 but it appears to be of doubtful generic importance, 

 since the generic characters should be mainly the more 

 fundamental internal ones. 



The intcrareas are well developed on both valves 

 and somewhat extravagantly on the ventral one. The 

 elongate ventral interarea, combined with the promi- 

 nent deltidium, might lead one to suspect that he was 

 dealing with a member of the Clitambonitida;, but this 

 is not true. The deltidium is strongly arched, especi- 

 ally at the wide end, and does not cover the del- 

 thyrium completely. The arching is greatest at the 

 open end of the deltidium, the margin of which is 

 strongly concave toward the beak. The apex of the 

 ventral shell in all specimens of A^. jestinata (Bills.) is 

 broken, suggesting that there was an open foramen 

 which is now filled with matrix, hence the destruction 

 of the beak in internal molds. In any event, the 

 actual margin of the foramen was not observed. It is 

 well to emphasize again that the deltidium of Nisusia 

 is well developed, since one might expect the del- 

 thyrium in so primitive an articulate brachiopod to be 

 a completely open one. We therefore see that in this 

 very early development of the deltidium and chilidium 

 there is great value in classification, as first pointed out 

 by Beecher. 



The interior of the ventral valve shows surprisingly 

 few characters, but this may be due to the thinness of 

 the valves. Walcott says that these shells have "dental 

 plates that extend outward, also inward, forming on 

 the inside a shallow spondylium." We saw no such 

 structures in the specimens studied, and in our opinion 

 Nisusia does not possess a pseudospondylium, or any 

 structure even remotely similar to a spondylium, nor 

 do dental plates occur in the genotype, A^. jestinata. 

 If such plates were present, the internal molds would 

 show as indentations or tracks representing the position 

 of the plates. Squeezes, however, do show slight thick- 

 enings along the delthyrial margins, which may indi- 

 cate incipient dental plates or the growth tracks of the 

 teeth. The explanation of all this is that Walcott's 

 reference to dental plates (pi. 100, fig. If) and a 

 pseudospondylium in A^. jestinata was inadvertently 

 made on a dorsal valve. The cardinalia of Nisusia 

 do, in some degree, resemble a cruralium (see his p. 

 728), and his "spondylia" in Nisusia may have been 

 incorrectly identified in dorsal valves. Traces of mus- 

 culature occur rarely in the ventral molds, but are too 

 faint to make out their nature. 



The internal morphology of the dorsal v.ilve of 

 Nisusia, like that of the ventral valve, is misunderstood. 

 Walcott has described a cruralium, but it may be ques- 

 tioned whether this structure exists in any of the Cam- 

 brian genera. The brachiophores are flattened plates 

 like those of BUlingsella or Eoorthis, making the noto- 

 thyrial margin, and are supported by adventitious shell 

 deposit that extends from the floor of the notothyrial 

 cavity. These plates are situated oblique to the dorsal 



