460 



TERMINOLOGY OF 



stance of the animals forming a calcareous tube for their 

 protection, and which tube, until the French naturalist ex- 

 plained its true nature, had been considered as the shell 

 itself. To this family belong the Teredo, and the Asper- 

 gillum or Water-pot shell, perhaps the most singular of 

 its class. These are truly bivalves, but the proper valves 

 are small, and their existence was long over-looked. In 

 Aspergillum the part generally preserved in collections is the 

 tube, to the inside of which, near its lower extremity, the 

 valves are closely soldered : but in Teredo the true shell is 

 placed without the tube at the posterior extremity. The 

 valves are small, and somewhat anomalous in form, while the 

 tube is long, flexuose, and worm-like, and lines the bore 

 which the creature has made in the wood. 



The annexed diagram (Fig. 84) shows the manner in which 

 a Bivalve shell is divided for the purpose of systematic 



Fig. 84. 



description : a, the apex or beak ; b, the position of the 

 lunule or areola ; cc, the superior or dorsal margin or slope, 

 with l f the ligament ; dd, the front margin or slope ; ee, the 

 posterior or siphonal margin or slope ; /, the inferior margin 

 or base ; g, the umbo or umbonal region ; 00, the longitu- 

 dinal, and xx, the transverse diameters.* 



* This is the nomenclature now universally employed, but with Linnaeus 

 and Lamarck our anterior slope would be the posterior, and vice versa. Mr. 

 Gray has correctly observed that it is impossible to understand the descrip- 

 tion of a bivalve shell " without taking into consideration the particular 



