BEEKMANTOWN AND CHAZY FORMATIONS OF CHAM PLAIN BASIN 409 



cause Hall's statement expressly implies that the form is very com- 

 mon in the middle Chazy beds, which is the case with the material 

 here referred to C. tenui septum. From more complete ma- 

 terial we derive the following description : 



Description. Large, slender orthoceracone, with closely arranged 

 septa and very large, ventren siphuncle. The conch attains at 

 least a size of ^ m as indicated by our largest specimen. Its 

 rate of growth is 13 mm in 100 mm; the largest section has a 

 diameter of 105 mm. The section of the conch is circular, with a 

 slight flattening on the ventral side, where the siphuncle is in contact 

 with the outer wall. 



The cameras are extremely shallow, 7 cameras being found in the 

 space of 20 mm. The septa are exceedingly thin, very concave, their 

 depth attaining thrice that of the cameras. The sutures are slightly 

 undulating; they are provided with a narrow, low, ventral saddle 

 and wider low lateral lobes. The living chamber is large ; its full 

 extent and the aperture have not been observed. 



The siphuncle is large, its diameter more than one third that of 

 the phragmocone in younger and fully one half of the same in older 

 specimens; circular in section, at least in the earlier stages; ventren 

 in position, though not in absolute contact with the outer side, for 

 the sutures of the septa form a high, sharp saddle upon the ventral 

 side of the siphuncle and a correspondingly deep lobe on the oppo- 

 site side [sec pi. 5 fig. 6] ; the septal necks reach apicad to the 

 preceding septum but no farther. A large (ephebic) portion of 

 the siphuncle remains continuously open, that of the earlier stages 

 is solidly filled with organic deposits, which exhibit distinct endo- 

 siphosheaths and an endosiphotube. A nepionic bulb [pi. 5, fig. 

 5l which had become entirely or mostly incorporated into the 

 phragmocone and hence is not visible from the outside is present. 

 It attains a greatest width of 13 mm and the siphuncle shrinks 

 directly anteriorly again to a diameter of 9 mm. 



The surface is marked with fine engirdling lines. 



Position and locality. This species is common in the dove-col- 

 ored Chazy limestone near Little Monty bay near Chazy village and 

 has also been obtained from dove-colored limestone (Q) of Yalcour 

 island and of Isle La Motte and from the Strephochetus bed (C G ) 

 at Chazy village; in the latter place in poorly preserved specimens 

 of somewhat doubtful reference. Raymond cites it from the beds 

 with M aclnrea m a g n a at t own Point and Hall's type came 

 from the dark Maclurea nrigna limestone of Chazy. 



