304 



distinct thereform, and that it is both generically and specifically identical 

 with the subsequently described Plectoceras ohscurum of Hyatt. 



The latter conclusion is based upon a direct comparison of the specimens 

 of P. Halli, collected by Mr. Weston, with the types of P. obscurum in 

 the Museum of Comparative Zoology at Cambridge, Mass., kindly lent to 

 the writer by Dr. W. Y. M. Woodworth, in 1903. As elsewhere stated,* 

 the types ' of P. obscurum are three in number, one a comparatively 

 perfect specimen from the Black River limestone at Watertown, N.Y., 

 marked 2077 ; and the others, two fragments from Watertown, each marked 

 2078. The specimen marked 2077 has nearly the whole of one side worn 

 away, but the other side shews the general shape of the shell and its surface 

 markings very well. It is about three inches and a half in its maximum 

 diameter and consists of two entire whorls. The inner whorls, if there 

 were any, are not preserved. Both sides of the specimen shew that the 

 whorls are at first so closely coiled that the inner half of the outer whorl 

 is in close contact with the one that immediately precedes it, but that its 

 outer half is free and slightly uncoiled. At the anterior end of the shell, 

 the outer whorl is about seventeen millimetres apart from that which im- 

 mediately precedes it. And it would seem to be the body chamber, which 

 occupies rather less than one half of the outer whorl, that is free and 

 separate. The surface markings are precisely similar to those of the fine 

 specimens of P. Halli collected by Mr. Weston at Lorette. On the worn 

 side all the septa but the last are obliterated, and the shape and position 

 of the siphuncle are not at all clearly shewn. A label, in Hyatt's hand 

 writing, however, states that the siphuncle is " marginal and ventral," as it 

 is known to be in P. Halli. The two fragments marked 2078 shew 

 neither the external form of the shell, the outline of the transverse section, 

 nor any of the surface markings. One of these is a little more than about 

 one-third of the outer whorl of a specimen which has been worn down in 

 such a manner as to shew a longitudinal section of the body chamber and 

 of the last five septa, which latter average from five to five and a half 

 millimetres in their greatest distance apart. The other shews scarcely 

 anything, except that the venter is much flattened. 



Adult or presumably adult specimens of P. Halli average about three 

 inches and a quarter in their maximum diameter, and appear to be always 

 smaller than full grown individuals of P. Jason. The former, too, are 

 more closely coiled, the rib-like folds on the outer volution are more 

 numerous and much narrower proportionately, while the siphuncle is quite 

 marginal. The characters of the external aperture are well shewn in 

 some of the specimens of P. Halli collected by Mr. Weston. The Up is 

 thin and simple, and its outer margin is exactly parallel with the flexuous, 



• Ottawa Naturalist, vol. XVII, pp. 161 and 162. 



