GENTTS PECTEN MFLLER. 4 I 



1ml ul:)ove continued on each side in a straiglit line to the unil)o. The shell adjacent 

 to these straight lines is frequently slightly ditlerent in sculpture from the rest of 

 the disk, forming narrow areas, which were called hy ('(niracl the 'submargins.' 

 Above the submargins the auricles or ears project, usually differentiated by a linear 

 depression ending in a sinus below. This sinus is sometimes al)sent in the posterior 

 ears, as it is in the very young stages of the shell, but it is not an important 

 systematic character, since the same species (e. g.. /'. Idtidnritus Conr.) may exhibit 

 varieties some of which have a well-developed posterior sinus while others are without 

 it. The right anterior sinus is usually emphasized by a Hexuosity in the lower edge 

 of the ear above it for the accommodation of the byssus, and on the upper part of 

 submargin are usually found a luimber of small, regularlj' spaced spines, which in 

 life separate the threads of the byssus and thus keep it from twisting with the motion 

 of the water. The growth of the margin of the valve and ear does not always march 

 with the development of these spines, so that a species which normally has them may 

 exhil)it stages when the valve margin has grown over the old set and the new set has 

 not been formed, much like the inequalities of growth shown by the niiirgin of the 

 aperture and the internal lirse of some (lasteropods. This set of spines, resembling a 

 short comb with curved teeth, has been called (^tenolium, pectineum, and pectinidium. 

 In old very heavy shells, which are held in place more by their own weight than by 

 the formation of a byssus, the^' are often absent, but may usually be traced in the 

 groove corresponding to the younger stages or fascicle of the sinus." 



The shell of Pecten, being thin in proportion to its surface, is in the adult 

 usuall}' ribbed or ffuted. This condition is brought about, doubtless, by natural 

 selection, and serves to strengthen the valves which, in swimming and falling to the 

 bottom, are often subjected to I'ude shocks. Shelly mattei- is sometimes de2)osited 

 internally in the shape of radiating lira? to still further strengthen the disk. 

 In addition to the ribs and riblets. the surface of Pecten may be ornamented by 

 fine, almost microscopic groovings, which radiate from the umbo (shown in P. 

 peckfuuid), or by a minute concentric sculpture due to imbricated incremental lines. 

 A combination of minute concentric and radial sculpture often gives the surface, 

 especially in (Jhlcuni/x. a checkered squamation or microscopic reticulation. 



"The original protot3'pe of Pecten, judging from the stages of recent shells 

 and the succession of the fossils, was a thin, nearly smooth shell, with a taxodont 

 provinculum and the posterior ears ill-defined." These characteristics have been 

 altered in the subsequent forms to meet conditions of environment, in some 

 instances certain features appearing and then becoming obsolete as their usefulness 

 decx'eased. 



"The discrepancy in size of the valves appears to be more or less related to 

 the activity of the animal; the species in which the difference is gi'eatest being 

 probably the more sedentary." 



P. excamatus is an example of a species in which the discrepancy is very 

 great, the right valve being extremely convex, while the left is somewhat concave. 

 Where a discrepancy is shown the right is usually the more convex valve, although 

 in some instances, notably in P. hoicerfii and /'. anderso)n, the reverse is true. 



The swimming habit is often exercised by Pecten, it being more common to 

 the thin-shelled, light, and young individual than to the thick-shelled, adult, and 



