222 Mr. Gray's Conchological Observations. 



Children, most admirably illustrate this fact; for the valves being 

 unequally convex, if the two shells are placed on their most con- 

 vex valves, their beaks turn towards each other, as the mouths of a 

 reversed and regular spiral univalve would do if placed on their 

 backs. This fact is most easy of discovery in the unequivalved 

 shells, where it often takes place, but it can only be discovered in 

 the equivalve ones, by an attentive examination of their teeth, 

 when the number and position vary in the two valves ; in which 

 case, as in the one above quoted, the teeth of the right valve will 

 be on the left, and vice versa. 



But here this curious circumstance, as in the univalves, often 

 takes place naturally, more especially in the attached and inequi- 

 valved shells, which are sometimes attached by one, and sometimes 

 by the, other valve, to which Lamarck refers when he divides his 

 Chamae into those which have the beaks turned to the right, and 

 those which have them turned to the left; when the fact is, that 

 they are only attached by the right or left valve ; but I have seen 

 two specimens of the same species, which were under the same 

 predicament as the two Lucinas Childrinae, and consequently one 

 of these shells was truly reversed. 



The symmetry of the two sides of the animal, and of the shell, 

 precludes this variety from taking place in the other shells. 



§ 13. On the Structure of Peneroplis and PoUontes, of 

 De Montfort. 



In my observations on the structure of the chambeied shells, I 

 attempted to explain the formation of the Nautilus Pompilius, 

 Spirula, Sfc. ; but in all these the shell ends in a cavity, which is 

 commenced by a concave diaphragm : but in some of the minute 

 species there is no terminal cavily, as the shell ends by a convex 

 diaphragm, which is usually perforated by a simple hole, a star, or 

 a series of minute foramina; and appears to be formed by a cer- 

 tain number of hollow cells, which are variously placed, sometimes 

 spirally, as in Peneroplis, and at others irregularly, as in PoUontes 

 which Montague called Serpula seminulum. I do not know what 



