UNITY OF DESIGN. 879 



around themselves in the form of a windmg tower, gradu- 

 ally diminishing towards the apex (PL 44, Fig. 14.*) 



The same essential characters and functions pervade the 

 Turrilites, which we have been tracing in the Scaphites, 

 Hamites, Baculites, and Ammonites. In each of these 

 genera it is the exterior form of the shell that is principally 

 varied, whilst the interior is similarly constructed in all of 

 them, to act as a float, subservient to the movements of Ce- 

 phalopodous Mollusks. We have seen that the Ammonites, 

 beginning with the Transition strata, appear in all forma- 

 tions, until the termination of the Chalk, whilst the Hamites 

 and Scaphites are very rare, and the Turrilites and Bacu- 

 lites do not appear at all, until the commencement of the 

 Cretaceous formations. Having thus suddenly appeared, 

 they became as suddenly extinct at the same period with 

 the Ammonites, yielding up their place and office in the 

 economy of nature to a lower order of Carnivorous mol- 

 lusks in the Tertiary and existing seas. 



In the review we have taken of genera in the family of 

 Chambered shells, allied to Nautilus, and Ammonite, we 

 have traced a connected series of delicate and nicely ad- 

 justed instruments, adapted to peculiar uses in the economy 

 of every animal to which they were attached. These all 

 attest undeviating Unity of design, pervading many varied 

 adaptations of the same principle; and afibrd cumulative 

 evidence, not only of the exercise of Intelligence, but also 

 of the same Intelligence through every period of time, in 

 which these extinct races inhabited the ancient deep. 



* The shells of the Turrilites are extremely thin, and their exterior is 

 adorned and strengthened (like that of Ammonites,) with ribs and tubercles. 

 In all other respects also, except the manner in which they are coiled up, 

 they resemble Ammonites; their interior being divided into numerous 

 chambers by transverse plates, which are foliated at their edges, and pierced 

 by a siphuncle, near the dorsal margin. (PI. 44, Fig. 14, a, a.) Tiie outer 

 chamber is large. 



