UiXIVALVE SHELLS. 



475 



II. SIMPLE UNIVALVES = UNIVALVIA ABSQUE SPIKA 

 REGULARI, LIN. 



Simple univalves are not twisted spirally round an axis ; 

 — they are unwreathed shells, which are tubular when they 

 taper to the apex and open at both ends ; and patelliform 

 when they are conical and hollow like a cup. 



Fio-. 93. 



Fio-. 94. 



The tubular shell (Fig. 93) belongs to a single genus 

 (Dentalium), that of itself constitutes an order in the 

 class. 



The conical shell (Fig. 94), although also pertaining prin- 

 cipally to a peculiar order of Gasteropods, is not restricted to 

 it. The shell embraces a wide series of similar forms. The 

 simplest is a low cone with the summit obtuse, and the mar- 

 gin entire or angular ; a second has the summit perforated ; 

 a third has a fissure on the posterior margin ; a fourth has 

 the apex pointed and recurved ; a fifth has within the hollow 

 a transverse partition ; and a sixth has a cup affixed under the 

 dome (Fig. 95). This is a very re- 

 markable structure. Professor Owen 

 assigns a reason for its creation in 

 the following passage. " The ne- 

 cessity for such a superaddition is 

 probably to be sought for in the more 

 active locomotive powers of Calyptraea 

 as compared with Patella ; the foot 

 in the former being, from its organiza- 

 tion, adapted to more extensive and 

 frequent contractions, would be liable 

 to affect the superimposed viscera if they were in imme- 

 diate contact with it. A calcareous plate, the first stage of a 



Fiff. 95. 



