CEPHALOPODA. 265 



Hamilton Group. 



Orthoceras erotalum. Hall. Several small fragments preserving the surface characters, which cannot be 



distinguished from this species. 

 " nuntium, " Fragments resembling this species have been observed from the limestone in 

 Ontario, Canada West. 



In addition to the species in the foregoing list there are eight forms known, 

 which constitute the characteristic species of the formation. Those described 

 in this work are as follows : 



Orthoceras Ohioense, 

 molestnm, 



jflrl/f mil, 



inoptatum, 



rtulh 



sirpus, 



riatiir, . 

 prof u ml n in. 



Hall 



" > Robust forms (Section a). 



M 



II 



J 



" ? 



• ,t ( Slender forms (Section 6). 

 " )■ Cone-fluted (Section d). 



Orthoceras molestum. 



FLATK XXXV, FIG. 13. 



Orthoceras molestum, Hall. Illustrations of Devonian Fossils: Cephalopoda. Explanation of plate 35, 



figs. 6, 8. 1876. 



Shell straight, rapidly enlarging from the apex. Transverse section circular. 

 Apical angle 9°. Initial extremity unknown. 



Chamber of habitation large, regularly and rapidly enlarging to the aper- 

 ture, having a diameter at the smaller extremity less than two-thirds of the 

 diameter at the aperture. Air-chambers regular, with a depth of nearly 

 four mm. ; the internal cast of the walls is smooth, and does not depart 

 from the general contour of the tube. 



Septa slightly indented by the siphuncle. Sutures distinct, straight and 

 horizontal. 



Siphuncle small, excentric, nearer the ventral side. Its other characters 

 not observed. 



The test is thin, having a thickness of .3 mm. Surface marked by broad, 

 lamellose, subimbricating lines of growth, which make a slight retral curve 

 on the ventral side over the siphuncle. 

 34 



