CEPHALOPODA. 99 



lus; and those with sutures, lobed and foliaeeous like the 

 fossil Ammonites. In the former the siphuncle is central or 

 internal {i.e., at the margin next the spire) ; in the latter it is 

 external {i.e., at the back of the shell, but ventral as regards 

 the animal, see fig. 29). There are, however, Nautili with 

 lobed sutures (Aturia, Bronn, fig. 28, i) ; and some with an ex- 

 ternal siphuncle (Cryptoceras, d'Orb.) And on the other hand, 

 the sutures of the Ammonite are at first very slightly lobed, 

 and become progressively more complex; so that specimens 

 of the same species have been referred to three genera — 

 Goniatites, Ceratites, and Ammonites — according to their 

 age. 



With the exception of Goniatites, the Ammonitidce are 

 peculiar to, and co-extensive with, the secondary strata ; while 

 the Nautilidw, with the exception of Nautilus and Aturia, 

 are confined to the palaeozoic rocks. But the palaeozoic so- 

 called Nautilidm exhibit peculiarities suggesting very wide 

 differences from the modern pearly Nautilus. It has been 

 proposed to associate the greater part of them with the Ortho- 

 cerata as a distinct family, but at present the data are defec- 

 tive. Like the Ammonitidce, their shells assume almost every 

 conceivable form and curvature, and the genera founded on 

 these characters are very ill defined. 



Nautilidm. — Some of the carboniferous Nautili have a 

 square back, and the whorls either compact or open in the 

 centre (fig. 30, i) ; whilst the last chamber is more or less 

 disunited. The species with the whorls quite disunited con- 

 stitute the genus Trigonoceras, M'C. ( = Nautiloceras, d'Orb.) 

 The Silurian genus Trochoceras, Barr, is a spiral Nautilus. 

 Clymenia, a characteristic Devonian fossil, has angular sutures 

 and an internal siphuncle; it may perhaps be coiled up 

 ventrally like the Spinda, The tertiary shell called Nautilus 

 zic-zac (Aturia, Br., fig, 28, i, 2), which is so widely distributed 

 in Europe, America, and India, has a siphuncle nearly mar- 



