NO. 1229. CAMBRIAN BRACUIOrODA—WALCOTT. 685 



discovery, despite the g-reat similarity of the two genera in other 

 respects. Except for the pedicle slit and chamber, the species of 

 Obolella could not well be taken from Obolus. 



OBOLUS AND ITS SUBGENERA. 



LinguIeUa Salter. I have been at times almost doubtful of the 

 advisability" of characterizing Lingulella even as a subgenus of Obolus. 

 This distinction is now based on the more elongate form of most of the 

 species of Lingulella and the greater thickness of the shell of the typ- 

 ical forms of Obolus. 



Type. — Oholus {LlvgultUa) davisil. 



Lingulejns Hall is an elongate, acuminate form of Lingulella, thus 

 departing most widely in form from Obolus. 



Type. — Oholus {Lingulejyis) acununatus. 



Linqidoholus Matthew is a Lingulella-like form, with a very thick 

 shell. ' 



Type. — Oholus {Lingulohohis) affinis. 

 ' Schmidtia Volborth is a Lingulella-like shell without radial striation. 

 All the species are small, and as the concentric stride are very line the 

 shell surface is nearly smooth. 



Type. — Oholus {Schrnldtia) celatiis. 



Wesfonia Walcott is a Lingulella-like form distinguished by peculiar, 

 transverse, semiimbricating, ''ripple-embossed" lines that cross both 

 the concentric and radiating strife. 



Type. — Oholus {We-'<toma) aurora. 



Thymnotos Mickwitz. An Obolus with strong, uniformly curved- 

 concentric strife, with lamellae of growth fringed along their anterior 

 (external) edges. 



Tyjje. — Oholus {Thyseuiotus) slluricus Eichwald. 



Acritis Volborth. Concentric lines elevated, irregular, undulating. 

 Valves strongly arched, massive. Visceral area (splanchnocoele) small 

 and short; pedicle furrow conical and deeply impressed in area. 



TyjM:. — Oholus {Acrit(s) antlqidsslmus Eichwald. 



Leptemholon Mickwitz. This subgenus is rather difficult to charac- 

 terize. Mickwitz says of it: 



The subgenus Leptenibolon is based ou a species of Obolus which externally 

 resembles Lingula very closely, and in fact was by earlier authors^ regarded as such. 

 The specimens of the internal surfaces of the valves, however, showed, together with 

 -nine suggestions of the last-mentioned genus (Lingula), unmistakable marks of the 

 genus Obolus, so that the species, which could not be assigned to any of the other 

 groups, had to be ranked in a special subgenus of Eichwald' s genus." - 



Typif. — (^hejlus {Lepteudxjlori) Ungulo'forrnis Mickwitz. 



^Schmidt, Fr., Ke\asion der silurischen ostbaltischen Trilobiten, Pt. I, p. 17. 

 ■^ tjber die Brachiopodengattung Obolus Eichwald, p. 199. 



