CEPHALOPODA. 305 



tions which have been made from this group, and without especial reference to 

 any particular class of fossils, it is very probable that future collections will 

 furnish a larger number of Orthocerata. The species here described seem to 

 possess few characters in common with the forms of this genus in other groups. 



Orthoceras Atreus, n. sp. 



PLATES I.XXXVIII, FIG. 1 ; LXXXIX, FIGS. 10, 11. 



Shell large, straight, robust, very regularly and gradually enlarging to the 

 chamber of habitation. Transverse section circular. Apical angle about 6°. 

 Initial extremity unknown. 



Chamber of habitation subcylindrical, well developed ; length about three 

 times the diameter of the tube at the last septum. Tube with a very broad 

 and gentle constriction anterior to the middle of the outer chamber, and an 

 indication of a contraction at the aperture. 



Air-chambers regular, deep, having a depth of fourteen mm. where the 

 tube has a diameter of fifty-five mm. The external walls are smooth in the 

 cast, with a slight longitudinal carina indicating the ventral side. 



Septa smooth, so far as observed, with a concavity equal to an arc of 120°. 

 Sutures straight and horizontal. 



The siphuncle, as determined from a small fragment referred to this spe- 

 cies with some doubt, is large .and excentric, having a diameter at the septa 

 of five mm., where the tube has a diameter of fifty mm., and distant from 

 the nearest margin of the septa about one-third the diameter of the tube. 

 It is expanded in the interseptal spaces, as indicated in a longitudinal section. 



The test is not preserved. The internal mould shows evidences of a trans- 

 versely lamellose-striate surface. 



A fragment, embracing a large portion of the chamber of habitation and 

 nine air-chambers, has a length of 300 mm., of which 160 mm. pertain to 

 the chamber of habitation. A smaller fragment, referred to this species and 

 preserving its normal form and proportions, contains nine air-chambers and 

 has a length of 115 mm. 

 39 



