BRACHIOPODA. 51 



fauna, dates as far back as the first introduction of the Trenton and Caradoc 

 faunas in America, thus preceding in time the linguloid forms of our series. 

 It thei-efore appears that this member of the group has received from some 

 earlier, and probably different source, the characters which make it the nearest 

 ally of Trimerella. Dinobolus is the only member of this family with an oboloid 

 form ; its shells were much lighter, and may prove to have been less calcareous 

 than in the rest of the group, not including Lingulops and Lingit'lasma among 

 the Trimerellidse as constituted by Davidson and King. 



The genus Obolus presents an interior so strikingly similar to that of Dinobo- 

 lus, that the first known species of the latter genus, D. Davidsoni, was for many 

 years referred to Obolus, by so acute an observer as Mr. Davidson himself The 

 essential difference, however, between Obolus and Dinobolus is precisely that 

 which we have just noticed between Lingula and Lingulops, i.e., the central 

 " spectacle-scars" in Obolus are excavated and not elevated, lying, as in Lingula, 

 at the anterior margin of the thickened visceral area of the shell, while in Dino- 

 bolus they are elevated and upon the platform. The successive transition stages 

 from the one form to the other are not so clearly demonstrated as in the line of 

 descent from Lingula. Attention, however, may be directed to the species de- 

 scribed as Oholella desiderata by Mr. E. Billings, the shell upon which Mr. S. W. 

 Ford has established the genus Elkania. This is a species which attains about 

 the size of Obolus Apollinis, Eichwald, and is larger than the Obolellas and Lin- 

 narssonias usually found in the associated faunas. In its brachial valve the cen- 

 tral and lateral muscular scars are elongated, as usually in the Trimerellids, rather 

 than localized as in Obolus, and are raised upon a well-defined platform, Avhich is 

 situated quite in the posterior or cardinal portion of the valve. The develop- 

 ment of the platform in this valve is considerably in advance of the progress 

 made in the opposite valve in the same direction, precisely as is the case in 

 Lingulops. The pedicle-valve, however, shows that the corresponding muscular 

 impressions are distinctly elevated, and, in front of the solid sub-triangular 

 plate on which they rest, is a very deeply depressed area occupying the central 

 portions of the valve and corresponding to the deep concavities having a 

 similar position in all the Trimerellids. A very conspicuous feature of this 



