GEOGNOSY AND GEOLOGY. 183 
family. At first they occur as completely chambered shells, with the windings 
in the same plane and in contact with each other ; in the cretaceous system, 
when about to disappear from the fossiliferous rocks, they begin to exhibit 
the original character of the Nautilide. The Crioceras of the neocomien 
come very near to the true Nautilus (see fossils of the Jura). Toxoceras 
and Scaphites of the same beds stand further removed, as also do the 
Hamites of the gault. The Baculites of the Ammonitide correspond to the 
Orthoceratites of the Nautilide: in the former, the chambers with fringed 
or sinuated margins are superimposed along a straight axis. P/. 39, fig. 43, 
represents Hamites attenuatus, and fig. 44, Baculites anceps, found in the 
middle chalks. Of the true Ammonites, A. varians (fig. 39) is found in the 
white chalk. The various forms of the Ammonitide just referred to, have 
had their turns all in the same plane. Turrilites has them in different 
planes, so called from being of a turreted form (7. costatus, fig. 45). This 
genus is distinguished from Helicoceras by having the turns in contact. 
Belemnites mucronatus (pl. 42, fig. 22) is known by the deep emargination 
of the base. 
The fishes of the cretaceous period exhibit a transition from the more 
ancient forms to those of the present day. Pl. 40, fig. 7°, represents the 
quadrangular broad teeth of Ptychodus latissimus, hitherto only found in 
the white chalk. The ganoids become less abundant, and in their stead 
come cycloids and ctenoids. The scales of Beryx are represented in 
pert. 
A gigantic lacertan reptile is found in the upper cretaceous group of 
Maestricht, Mososaurus hoffmanni, as also some crocodilians ; the cretaceous 
of North America, likewise, embraces species of Mososaurus. The 
Maestricht beds also contain teeth of [guwanodon mantelli (fig. 11"). But 
few remains of birds occur, a partial skeleton is shown in fig. 12. 
Other fossils, characteristic of the English cretaceous, are Pecten 
quinquecostatus (pl. 42, fig. 13); Aptocrinus ellipticus (fig. 14); Spongia 
cribrosa (fig. 15); Marsupites milleri (fig. 16); Inoceramus sulcatus (fig. 
17); Trigonia aleformis (fig. 18) ; Cotillus brogniartii (fig. 19) ; Ammonites 
varians (fig.20); Plagiostoma spinosum (fig.21) ; Belemnites mucronatus 
(fig. 22); and Scaphites costatus (fig. 28). 
Tertiary Middle Series. 
In ideal succession, the tertiary middle series lies immediately above the 
newest deposits of the secondary middle. This, however, does not always 
occur in nature, and although the tertiary beds are not found between the 
older rocks, they yet at times rest upon them, and even upon abnormal 
masses, by which they are not unfrequently pierced and overlaid. ‘ the 
latter case, the abnormal must be the newer of the two; it is generally 
basalts that present themselves among the tertiary in this manner. 
The thickness of the tertiary is sometimes very considerable, as, for 
instance, on the Righi, 6000 feet high; they also occur at considerable 
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