NOTE ON A SPECIMEN OF PLECTOCERAS JASOX 



(BILLINGS) 



(With one plate) 



BY RUDOLF RUEDEMAXX 



Preparations for the moving of the State ^Museum to the new 

 Education Building brought to light a block of middle Chazy lime- 

 stone from Valcour, Clinton county, N. Y., collected by Professor 

 van Ingen and the writer, which had not been at hand when the 

 Cephalopoda of the Beekmantown and Chazy formations of the 

 Champlain Basin were described by the writer (N. Y. State Mus. 

 Bull. 90, 1906), but which is so much superior in perfection of shell 

 and in size to the specimens used for that paper that we are sure 

 the publication of its figure will be an addition to our knowledge of 

 the form. Above all it shows the entire living chamber and the 

 apertural margin, both not observed before, as far as we are aware. 



The living chamber attains in length a little more than one-third 

 of a volution. It is entirely free and furnished in the specimen with 

 very strong costae except near the aperture where the ribs cease 

 rather abruptly. The aperture is directed as it had been described 

 from the growth lines in the above-mentioned paper. 



We have drawn in the nepionic and neanic portions observed in 

 another specimen and figured in longitudinal section in the former 

 publication,^ thus obtaining a view of the entire specimen. The rate 

 of growth of the conch is thereby distinctly shown. 



The uncoiled condition of the entire living chamber and the dis- 

 appearance of the ribs near the aperture suggest that the specimen 

 had attained mature if not gerontic age, and the shell therefore 

 represents the full size of this species. 



The septate portion of the whorl here represented is exfoliated 

 and therefore, while showing the septa, fails to show the costae 

 which are much closer in the neanic shell. The surface of this 

 portion is represented in Bulletin 90, plate 29, and indicated in the 

 restored part. The living chamber is also largel}^ exfoliated and only 

 shown in its interior cast and the smoother character of the apertural 

 portion may be largely due to this state of preservation. The last 

 portion of the apertural chamber would then seem to have possessed 

 a thicker wall. On account of the absence of the shell, the conch 

 now also appears more evolute than it was in reality. 



^ Text figure 44, page 485. The figure is in natural size, and not X ^ as 

 erroneously marked. 



