CONCHIFERA. 



709 



Fig. 364. 



Pholas. 



Petricola. 



these appendages are the first 

 parts of the cardinal teeth. There 

 is one fact which deserves to 

 be insisted on in connexion with 

 this genus ; it is that there are 

 no ligaments found, and that 

 the cardinal edge, folded in 

 upon itself (rentre sur lui- 

 rnn/te), is not flattened and 

 placed in the same manner as in 

 the other conchifera. Another 

 circumstance of equal impor- 

 tance to be mentioned is that 

 the processes, of which we have 

 just spoken, are buried in the 

 substance of the animal, and 

 covered with a duplicature of 

 the mantle which accompanies 

 them as they plunge amid the 

 visceral mass. Without leav- 

 ing the genus Pholas, the cuil- 

 lerons may be seen gradually 

 contracting in their breadths, be- 

 coming shorter, and approaching 

 c nearer and nearer to the edge. 

 But if other shells be examined, 

 which obviously form the links 

 of transition from the Pholada to 

 the Saxicava, or Petricola, the 

 processes are found to turn upon the edge, to 

 become coherent with it so as to form a salient 

 margin, and by their free extremity to produce 

 a projection (/>, fig. 364). In our opinion the 

 toothings of the hinge of all the other bivalve 

 shells are produced in the same manner; but 

 with such modifications as rarely admit of those 

 relations being traced which are to our mind 

 obvious in those genera that have just been 

 particularly mentioned. With regard to the 

 shells of the genera in which the hinge is 

 complicated, of which the cardinal edge is 

 thickened, and the cavity of the hook partly 

 filled by the external layer of the shell, it is 

 difficult to imagine in what manner the suc- 

 cessive growth of the hinge has taken place, 

 and to make out its analogy in point of struc- 

 ture with that of the Petricola pholadiformis 

 and of the Pholada generally. To discover 

 this it is necessary to break a great number of 

 the shells, or to make various sections of the 

 edge, vvhen the direction of the denticulations 

 with which it is furnished must be followed. 

 The teeth of the hinge will then be seen arising 

 from the summit of the hook (c, Jig. 364), 

 becoming developed, and forming a solid arc, 

 surrounded and hidden by the matter of the 

 cardinal edge itself, and these arcs thus disen- 

 gaged will be found to present the strongest 

 analogy with those of the Pholada. It is from 

 viewing the hinge in this manner that we have 

 been induced to think that its structure was in 

 reality of sufficient importance to make it be 

 constantly appealed to for the distinguishing 

 characters of genera. 



Naturalists have agreed to designate as the 

 cardinal teeth those solid projections which 

 arise on the edge of the hinge. These projec- 

 tions on the one valve are for the most part 

 accompanied with corresponding depressions 



on the other for their reception mutually. The 

 depressions are called cardinal pits. These 

 cavities and these projections present a great 

 variety of modifications which cannot be well 

 understood without a long and careful study of 

 the conchiferous tribes generally. When the 

 teeth are collected under the hook, they pre- 

 serve the title of cardinal (b, Jig. 365) ; when 



Fig. 365. 



one or two in number, and remote from the 

 centre of the hinge, they are named lateral 

 teeth. Of these lateral teeth one is an- 

 terior (c, Jig. 365), the other posterior (d y 

 fig. 365). The anterior lateral tooth is com- 

 monly situated at the extremity of the lunule, 

 and the posterior lateral tooth at the extre- 

 mity of the ligament. The cardinal teeth, 

 properly so called, vary in number. When 

 there are but two, the one is anterior, the other 

 posterior ; when there are three or more, those 

 in the middle are entitled median teeth. If 

 the hinge be composed of a great number of 

 teeth, it is said to be serial (6, 6, fig. 36). 



Fig. 366. 



Area. 



