56 



a small orifice surrounded with small plates— the anal opening 

 of some authors, and the mouth of others. Beneath the plates 

 of the vault there are channels passing- between the depressions, 

 meeting centrally, and connecting with each of the prolonga- 

 tions; tw r o small channels enter the bifurcating prolongations, 

 but only one is extended through the single branches, and it 

 does not open externally. It will be observed, that the marginal 

 plates of the vault are continued upon the lateral sides of the 

 bifurcations, except in the case of the single prolongation, and 

 it is evident the marginal circulation entered these prolonga- 

 tions, and also connected with the central circulation, through 

 the channels above referred to. The functions performed by 

 these lateral prolongations are, therefore, not essentially differ- 

 ent from those performed by the proboscis in other genera. 

 There is a slight elevation, within the margin, opposite the sin- 

 gle prolongation, and from this point only a single channel 

 seems to have entered the prolongation or unyoked feature of 

 this vault. The arms connected with the body immediately 

 below the rim of the vault, and some of the fragments of the 

 arms are preserved on our specimen, though not illustrated. 

 They are small and composed of a double series of interlocking 

 plates bearing small pinnules. 



This species is distinguished from others known to the authors 

 by the spreading of the bifurcating prolongations, instead of 

 their curving around, so as to cross each other, and by the 

 single extension which is not known, by us, to exist in any 

 other species. It is so different from the type of this genus, 

 and from all defined species from rocks of the same age, that 

 no comparison with any of them is necessary. 



Found in the Keokuk Group, on Indian Creek, near Craw- 

 fordsville, Indiana, and now in the collection of Win. F. E. 

 Gurley. The specific name is in honor of one of the authors of 

 the genus, the late distinguished palaeontologist, Sidney S. Lyon. 



