GENERA OF THE SUBORDER ORTHOIDEA 



49 



B. nutjor Walcott 1905 

 B. flualflla WiXcon 1905 

 B. strUlj Walcott 1905 

 Orthii fefina Hall 1863 



Clitambonites flanus retroflexus Matthew 1896 (looks much 

 like B. linJstromi) 



European Species 

 Billingsella lindstromi (Linnarsson) 1876 



Distinguishing characters. — The important 

 distinguishing features of B'dlingsella are the subquad- 

 rate outh'ne and biconvex lateral profile; ortho- or 

 slightly apsacline, elongate and plane interareas; large 

 teeth with low supporting lamcll<-e; prominent muscle 

 marks with elongate adductor track; strong pallial 

 trunks; primitive cardinalia; and the presence of a 

 deltidium and chilidium. 



Composition of genus. — Before we proceed with 

 the generic discussion, it should be said that Walcott in 

 1912 referred 20 species and 1 variety to Billings- 

 ella — a rather heterogeneous lot — and of these 14 are 

 found in North America, the remainder in Norway 

 and Sweden (3), Wales (1), Bohemia (1), and 

 China (2). Of these 21 forms we leave but 8 (?7) 

 in Billingsella (7 in America and 1 in Sweden). Of 

 the other 13, 2 are referred doubtfully to Ntsusia 

 {hivia and orientalis), 3 to Oligomys {exforrecta and 

 rugosicostata of Sweden and hicks't of Wales), 1 to 

 W'tmanella {highlandensis) , 1 to Bohemiella (romin- 

 geri) and 1 (5. dice Walcott) to Deltatreta; this 

 leaves 5 unplaced for want of knowledge of their 

 interior characters. It is at once apparent, therefore, 

 that our characterization of BiUingselln is narrower 

 and more precise than that of Walcott. 



Discussion — It is important, therefore, to define 

 precisely the morphology of B'dlmgsella, since the mem- 

 bers of this genus and especially the genotype, together 

 with B. coloradoensis (Shum.), have been widely fig- 

 ured and variously interpreted. Externally Bdlingsella 

 is distinctly subquadrate in outline, but more peculiar is 

 the very long interarea, wide delthyrium, and, in the 

 genotype and B. coloradoensis at least, the prominent 

 apically perforated deltidium. The deltidium is a dis- 

 crete convex plate, but usually less convex than Wal- 

 cott figures it on his plate 85, figure In, a figure which 

 unfortunately has been republished very widely; it is a 

 normal delthyrial covering that is apically perforate, 

 and in situation and size this aperture recalls that seen 

 in the deltidium of RafinesquirM or Strofhometui more 

 than that of the Clitambonitida:. Further, the del- 

 tidium of Bdlingsella is clearly an arch built with its 

 piers fastened to the inside of the walls of the delthy- 

 rial cavity just below the delthyrial margins of the 

 interarea. 



In the ventral valve the internal structures are dis- 

 tinctly orthoid or clitambonitoid. The dental plates are 

 widely divergent, obliquely cemented to the floor of 



the valve, and at their anterior ends are continuous 

 with the teeth. The pseudospondylium, when devel- 

 oped, is a callus under the muscle attachments, and 

 none of the species here referred to Bdlingsella has 

 excessive thickenings at the posterior end. Walcott 

 mentions such structures only in B. dice (pp. 749, 

 754), B. orientalis (p. 307), B. exporrecta (p. 307), 

 and B. highlandensis (p. 307), but all of these species 

 are referred by us to different genera. In Kozlowski's 

 opinion,* Billingsella has rudimentary spondylia, but the 

 type of dental plates and muscle area corresponds to 

 his spondylium discretum, which, however, is in reality 

 not a spondylium. 



The ventral musculature of Bdlingsella is essentially 

 orthoid in plan and its most important characteristic is 

 the elongate adductor track which expands as it grows 

 forward and is usually slightly elevated on a low callus 

 at its anterior end. The diductor tracks are shorter 

 and continuous with the pallial trunks. Positive evi- 

 dence of adjustor marks is lacking, as in Orthis s. s., 

 Hesperorthis, Nicolella, etc. These muscles may have 

 been confined to the dental plates, where they are not 

 visible, or may have been united with the diductors. 



The ventral musculature of several species presents 

 some variations from the normal type. In B. resseri 

 and B. trempealeauetisis (both of Ira Edwards MS.), 

 the diductor tracks unite for nearly the whole length 

 of the delthyrial cavity, restricting the adductor track 

 to a small subpentagonal scar at the antero-median 

 sides of the diductor marks. A similar arrangement is 

 suggested in B. highlandensis (a Wimanella) and 

 Wimanella simplex (see Walcott, pi. 87, fig. 4b, and 

 pi. 89, fig. 2d). 



One of the striking characters of Billingsella, and 

 one which has been exploited considerably, is the prom- 

 inent ventral pallial marks usually called "vascular" 

 markings. These consist of two more or less strongly 

 impressed, subparallel, narrow depressions, originating 

 at the front ends of the diductor scars and extending 

 antero-laterally. Near the front of the shell the sub- 

 parallel trunks fork, the outer branch curving back- 

 ward and extending nearly parallel to the periphery 

 of the shell and terminating near the cardinal extrem- 

 ity. The inner sinuses run obliquely toward the middle 

 of the shell, where they either die out or unite at the 

 mid-line, forming an elongate pentagon. Near the 

 front margin many subsidiary branches radiate from 

 the main channel. Pallial marks similar to those of 

 Billingsella are also common to other Cambrian shells, 

 and this fundamental type can be seen, with variations, 

 in all higher genera. 



The cardinalia are very distinctive and exceedingly 

 important from a taxonomic point of view. The 

 brachiophores are flattened plates set obliquely beneath 

 the interareal margins of the notothyrium. They are 



"Pal. Polonica, vol. 1, 1929, p. 128. 



