844 DIVISIONS OF THE CEPHALOPODA. 



the Cambrian to the Carboniferous, and attains its maximum in the 

 Silurian period, more than three hundred species having been de- 

 scribed by Barrande from the Silurian deposits of Bohemia alone. 



Family 8. Lituitid^e. — The shell (fig. 757) in this family has 

 its earlier portion coiled into a flat spiral, while the later portion, 



including the whole or the greater part 

 of the body-chamber, is produced in a 

 straight line. The volutions of the 

 spiral part of the shell may or may 

 not be in contact, and the siphuncle 

 is subcentral or internal. The septa 

 are simple and concave, and the sur- 

 face is transversely striated or ribbed. 



In Lituites itself the uncoiled por- 

 tion of the shell is very long, and the 

 aperture is contracted, with a deep 

 ventral sinus. The genus is found in 



Fig. 757- — Lituites cornu-arietis. ^ r\ i • ' j o-i • i 



Ordovician. the Ordovician and Silurian rocks. 



The Silurian genus Ophidioceras re- 

 sembles Lituites in most respects, but the disjunct portion of the 

 shell is short, and contains a portion only of the body-chamber. 

 Lastly, in the genus Discoceras, also from the Silurian, the terminal 

 and produced portion of the shell is short, but the aperture differs 

 from that of Ophidioceras in being simple. 



Family 9. Trochoceratid^e. — In this family the shell is rolled 

 up into a spiral, the coils of which do not lie in a single plane. In 

 Trochoceras the shell is unsymmetrically coiled, and the last volu- 

 tion may be partially disjunct. The septa are concave, and the 

 siphuncle is usually placed between the centre and the convex ex- 

 ternal margin ; while the aperture is simple. The genus ranges 

 from the Ordovician to the Devonian, but the great majority of the 

 species are Silurian. The Silurian Adelphoceras differs from Trocho- 

 ceras in having a contracted aperture to the shell. 



Family 10. Nautilid^. — In this family the shell is spirally in- 

 rolled, the coils lying in one plane, and the aperture being simple 

 or contracted. Of the numerous generic types included in this 

 family, the following are the most important : — 



In the Ordovician genus Trocholites the shell is discoidal and 

 umbilicated, and the septa are concave, the suture -lines being 

 simple or feebly lobed. The siphuncle is sub-marginal, placed 

 towards the internal side, and the septal " necks " are long, and 

 reach from one septum to the next. 



In Gyroceras (fig. 758) the shell is in the form of a flat spiral, the 

 coils of which are not in contact with one another. The aperture 

 is simple, with a ventral and dorsal sinus ; the sutures are simple ; 



