CEPHALOPODA. 51 



The septa in the Clymenida arc distinguished by lateral rounded or angular lobes ; 

 but the singular form is not peculiar to the family ; since, as we have already seen, it 

 is found in Nautilus Parkinsoni, a species which, possessing an excentric siphuncle, 

 must be considered as merely an aberrant form of Nautilus ; and the separation of the 

 Clymenida! will depend entirely on the siphuncle being placed on the dorsal margin. 



The two genera which belong to this family are distinguished chiefly by the mode 

 of involution of the shell ; the whorls in Clymenia being exposed, while in Aturia the 

 last whorl conceals the rest ; they therefore bear to each other the same relation which 

 Plamdites bears to the true Nautilus. 



Genus 5th. Aturia.* Bronn, 1837. 



Gen. desc. A. testa discoided vel subventricosd, spirali, midtiloculari, parietibits 

 simplicibus ; anfractibus contiguis, ultimo alios obtegente ; umbilicis clausis ; scptis trans- 

 versis, numerosis, extiis concavis, utroque latere angulariter lobatis et parte dorsali, magna 

 siphone infundibuliformi, marginibus positd, retro prolongatis ; mar ginibus simplicibus. 



Shell discoidal or subventricose, spiral, multilocular, sides simple ; whorls con- 

 tiguous, the last concealing the others ; the umbilicus closed ; septa transverse, 

 numerous, concave outwardly, with an angular lobe on each side, and having the 

 dorsal part prolonged backwards, forming a large, marginal, funnel-shaped siphon ; 

 margins of the septa entire. 



The angularly-lobed septum which distinguishes Nautilus Parkinsoni also forms a 

 prominent character in the well-known Highgate fossil, Naut. zic-zac, figured and 

 described by Mr. Sowerby in the first volume of the ' Mineral Conchology.' Bronn, 

 in his description of the Dax shell Nautilus Aturi (Bast.), which he considered to be 

 distinct from N zic zac, suggested the propriety of forming a sub-genus, to be called 

 Aturia, for the reception of those tertiary Nautili in which, according to the sub- 

 generic description given by him, "the siphon is sub-ventral (i. e. sub-dorsal), and the 

 septa have a deep, narrow, lancet-shaped flap on each side." The siphuncle, however, 

 in the Dax shell, on which the genus is founded, is, in fact, strictly marginal ; it is, as 

 Bronn himself describes it, a prolongation backwards of the dorsal part of the septum, 

 in the shape of a wide-mouthed funnel, extending quite across the preceding chamber, 

 and deeply into the mouth of the preceding funnel. As this funnel-shaped siphon 

 decreases in diameter, the dorsal paries gradually recedes from the margin, and the 

 intervening space is filled up with a calcareous deposit. The siphuncle, therefore, will 

 in some parts of its extent appear to be sub-marginal only : whereas the mouth of the 



* Etym. Aturrus vel Aturus — the River Adour. 



