440 



PECTEN, LIMA, AND AVICULA. 



into an organ which is termed the foot. The shape of the shell follows 

 that of the animal, and becomes circular, oval, and more and more 

 elongated transversely as the foot and digestive and respiratory organs 

 become more modified in form. We may perhaps illustrate the nature 

 of these modifications by examples. The common scallop is referred to 

 the genus Pecten. The characters which are common to all species 

 of Pecten are the definition of the genus, which the palaeontologist 

 needs to remember ; they are three. First, a straight hinge-line ; 

 secondly, two lateral processes of shell called ears, at the sides of the 

 central ligament of the hinge ; and third, the axis of greatest growth 

 at right angles to the central line of the hinge. No matter what 

 other characters the shell might have, if it had 

 these it would be a Pecten. The other characters 

 would serve to distinguish species. They would 

 be first, the relative length and breadth of the 

 shell ; second, the degree of inflation of its 

 valves ; third, the form and development of 

 the ears; fourth, size; fifth, the external con- 

 dition of the surface of the shell, which might 

 be smooth, marked with many kinds of radiat- 

 ing ornament, or concentric ornament, or can- 

 cellate ornament, &c. ; so that it is scarcely 

 conceivable that any external specific character should approximate 

 a pecten towards another genus. Yet a modification of one of the 

 generic characters would permit of that character being associated 

 with the whole series of secondary modifications of growth which we 

 agree to designate specific. Thus, if while the hinge-line remains 

 straight, as in Pecten, it is shortened so that the ears become smaller, 

 and the axis of growth is inclined anteriorly, so that the anterior 

 margin of the shell is straight or nearly so, and the posterior margin 

 convexly rounded, a generic type results, which is 

 indicated by the name Lima. Similarly, if the 

 hinge-line is elongated by extending the ears, and 

 the axis of growth is inclined posteriorly, the 

 genus Avicula is defined, and it too may undergo 

 variation, which is practically infinite, by modifica- 

 tion of ornament and form. From Avicula many 

 other genera may be derived ; thus, by still further elongation of the 

 hinge-line and increased obliquity of the axis of growth of the shell, the 

 ears become entirely obliterated, forming the genus Pinna. There are 

 some shells which, instead of growing only at the occludent margin, 

 also grow at the hinge margin ; and if we conceive of a form of Avi- 

 cula in which the ears are small, and in which the shell is marked by 

 fine radiating ribbing which reproduces itself in the hinge, as may be 

 seen in thin shells like some species of Modiola or Crenella, then the 

 step may easily be conceived by which a transition is made from Avi- 

 cula to the genus Area, characterised by its straight denticulated hinge- 

 line and transverse and inflated mode of growth. Area, through some 

 of its fossil representatives, like Macrodon of the Great Oolite, repro- 

 duces the hinge-teeth of the genus Avicula, and Area, by losing its ear- 



Fig. 81. Avicula. 



