376 



CAMBRIAN BRACHIOPODA. 



In Obolus the parietal band on the dorsal valve extends still farther toward the frontal edge than in Lingula, and in 

 its posterior part is more markedly bent inward from both sides, producing a characteristic unequally three-lobed 

 figure [fig. 34B]. The parietal band of the ventral valve of Obolus on the contrary is subelliptical in form, and rather 

 approaches that of Lingula. 



More important than this difference in form of the splanchnoccele is the difference in the form of the mantle lobes 

 which is manifest from the position of the posterior part of the parietal band. In Lingula the parietal band is moved 

 away from the base of the area and thus constitutes a narrow space between the two pleurocoeles, which space is 

 occupied by the mantle lobes that extend around the entire beak part of the valves. These mantle lobes of the beak 

 are in the dorsal valve also covered with mantle bristles, while the border of the mantle of the ventral valve in the 

 splanchnocoelic part of the area (deltidium. King) is free from bristles [fig. 34C]. In Obolus on the contrary the pos- 

 terior part of the parietal band is close to the base of the splanchnocoelic part of the area, whose lamellse, as we have 

 seen, are bent up at right angles to the plane of the valve and therefore could not have been deposited by mantle 

 lobes resting against the valves. Hence the mantle lobes of Obolus extended only as far as the pleurocoeles, and were 

 lacking, as well as the mantle bristles, in the splanchnocoelic part of the area of both valves. At that point there was 

 only the muscular wall of the body connecting the two valves, from which the pedicle emerged. 



I think that the differences pointed out are sufficient to distinguish Oiolus as of distinct 

 family relations from Lingula, but I must at the same time admit that the transitions from 

 Oholus to lAngulella and from Lingulella to Lingula are so clearly indicated that it is only by a 

 somewhat arbitrary decision that species of Lingulella like L. hella (Pis. XIX and XXXVI) can 

 be referred to the Obolidse. Schuchert [1893, p. 360] creates the family Lingulellidse ior Lingulella 

 and allied forms. 



Oholus and Oholella. — ^Authors have compared Oholella with Oholus, and Mickwitz [1896, p. 

 129] thought that they might possibly be congeneric. The discovery of a pedicle opening at the 

 beak of the ventral valve of Oholella places the latter with the Acrotretidse. 



Criteria of suhgenera. — The criteria upon which the subgenera of Oholus have been founded 

 are largely based on the form of the outline of the valves, character of outer surface, thickness 

 of shell, and, to a more limited extent, the muscular and vascular markings on the interior of 

 the valves. I was at first inclined to give considerable weight to the position of the sinuses 

 occupied by the main vascular canals, but further study showed such a gradation in the various 

 species that efforts to use the character in subgeneric classification were abandoned. The 

 followiag table illustrates some of the variations in the position of the vascular sinus, and many 

 more may be found in the figures of Oholus and its subgenera than are indicated in the table 

 below : 



Variation in the position of the vascular sinus of Obolus and its subgenera and Lingulella. 

 [Roman numerals indicate plates and arable numerals figures.] 



