PHACOCERAS OXYSTOMUM. 109 



Internal casts sbow the septa strongly arching forwards from the umbilicus to a 

 flat, solid band, about two or three lines wide, produced by the internal cavity 

 not quite reaching the edge ; the last chamber [body-chamber] occupies rather 

 less than half the last whorl, and is marked by a narrow, deep sulcus, a little on 

 the inner side of the middle, produced apparently by a corresponding ridge on the 

 interior of the shell (PL XXVIII, fig. 3 a),^ of which there is no trace on the 

 outside. ... I have not been able to observe the siphuncle in this species, 

 but according to de Koninck it is very small and nearly central, a little outside of 

 the middle. The inner whorl is scarcely embraced by the succeeding one, is not 

 compressed, and has a broad, convex periphery; the next has a much more acute 

 periphery, and is correspondingly embraced by the succeeding turn." (M'Coy, 

 ' British PalEeozoic Fossils,' fasc. iii, 1855, p. 560.) 



I am indebted to the kindness of my friend Mr. G. C. Crick, of the British 

 Museum, for the measurements given below, and he has also indicated to me by 

 means of a diagram the exact position of the siphuncle, which he ascertained by 

 temporarily separating the two halves of the specimen which had been united by 

 an adhesive. It proves to be nearer the centre of the septum than de Koninck 

 represents it to be, — that is, the height of the septum (the one measured was the 

 last) being 35*5 mm., the siphuncle occurs at a distance of 20 mm. from the dorsal 

 or wider extremity, and consequently 15*5 mm. from the peripheral or narrower 

 extremity of it. 



Dimensions. 



Specimen in " Gilbertson 

 Coll.," British Museum. 



Diameter of shell (without the test) . . .85 mm. 



,, umbilicus (with the test) . . 23 ,, 



Greatest thickness of shell (without the test) measured at 



the anterior end of the body-chamber, below the 



curved line . . . . . 15 ,, 



The same at the last septum . . . 13"5 ,, 



Depth of last four chambers (No. 1 being the last) as 



follows . . . . (1) 2-25 „ 



(2) 2-75 „ 



(3) 3 



(-t) 3 „ 



Affinities. — There is really no species which will bear comparison with the 



present one, which thus holds a solitary position among Palaeozoic Nautiloids. 



De Koninck's comparison of it with the Nautihis comijloAiatus of J. Sowerby 



(' Min. Conch.', 1821, vol. iii, p. 109, tab. cclxi) is not justified by the possession 



^ Seen ou the cast of the body-eliamber as a curved Hue following tlie contour of the shell. 



