NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 109 



lar scar may also be cited as furnishing some grounds for a subgeneric distinction under 

 Pterinea. This form may be compared with Avicula Triton Salter (Mem. Geol. Surv. Great 

 Brit. Vol. II. Pt. I. PI. xxiii. fig. 5.) 



(d) Pleurotomaria Axion Hall, has been referred by us to P. Hold Hall. We have, how- 

 ever, in our notice, cited the disagreements, while, in the absence of authentic specimens 

 of P. Halei, we hesitated to found a new species on the Bridgeport specimens. Having 

 now seen Prof. Hall's types of that species, an opportunity for which we would again 

 express our obligations, we are convinced that the separation of the Bridgeport speci- 

 mens is perfectly proper. 



(e) Tremanotus Hall, is a new subgenus of Porcettia, founded upon our Bellerophon (Bucania) 

 perforatus. We may have sufficient evidence of the existence of dorsal perforations in 

 this species. Nevertheless, the perfectly symmetrical enrollment of even the young shell, 

 as well as the enormously expanded aperture (not seen by Hall), would seem to indicate 

 stronger affinities with Bellerophon than with Porcellia. 



6. The following five new species described by us, have been also described by Hall. 



Ittcenus worthenanus W. and M. (I. insignis Hall). Acidaspis Ida W. and M. (A. Danai 

 Hall). Clidophorus macchesneyanus W. and M. (Modiolopsis rectus Hall). Bellerophon (Bucania) 

 perforatus W. and M. (Tremanotus Alpheus Hall). Gomphoceras Marcyce W. and M. (Gom- 

 phoceras scrinium Hall). . 



Of these, the following are quoted only from Bridgeport : Acidaspis Danai, Tremanotus 

 Alpheus, Gomphoceras scrinium. 



Illcenus armatus Hall, has been detected among our specimens since the appearance of 

 Prof. Hall's paper. 



Ct/pricardinia arata Hall, has also more recently been observed. 



Avicula undata Hall. Specimens have been brought to light which resemble this species, 

 and it is probable that they are the forms referred to it by Hall. They differ, however, in 

 having the beak su-bterminal, and the posterior wing not isolated from the body of the 

 shell and not extending as far back as the most projecting part. The right valve has two 

 posterior, linear, diverging teeth, which terminate abruptly three fifths the distance from 

 the beak to the cardinal extremity. We should feel inclined to separate these forms from 

 A. undata Hall. For the present we designate them as Pterinea undata Hall, sp. 



Modiolapsis subalata? Hall. We adopt nearly the same observations in reference to 

 certain other forms, which probably are the ones referred by Hall to Modiolopsis subalata. 

 Our specimens, however, have, in the right valve, two linear, posterior teeth, and two short 

 lamelliform anterior teeth. The size of the shell is also more than twice that of the 

 types of the species. It seems to possess the characters of a true Pterinea. The left valve 

 is much more ventricose, and shows distinctly a strong arcuation of the body, which is 

 less visible in the thinner right valve. Casts of both valves show a few obscure, remote 

 radiating ridges on the anterior slope. 



The following new species have also been brought to light by recent work in the 

 quarry : 



8 



