PANAMIC-PACIFIC PELECYPODA 4S 



or the posterior one is much larger (Heteromyarian). In some other forms, 

 such as the scallops and oysters, there is but one adductor scar (Monomyar- 

 ian); observation will show that this scar is not exactly in the middle but 

 nearer the posterior side and hence is an aid in deciding whether the valve 

 is right or left. The pallial line, already mentioned, is a more or less con- 

 tinuous linear scar or series of small impressions marking the line of attach- 

 ment of the mantle to the valve. It is generally placed close to the ventral 

 margin. The pallial line may be developed as a simple, curved Une between 

 the adductor scar (an entire pallial line), or it may show an inflexion at the 

 posterior end (pallial sinus). A large pallial sinus indicates that the clam 

 possessed large, extrudable siphons such as in Mya. 



ARTIFICIAL KEY TO THE PRINCIPAL FAMILIES OF 

 PANAMIC-PACIFIC PELECYPODA.-* 



I. Monomyarian. Each valve with a single adductor scar only; this scar 

 is placed slightly behind or posterior of the middle. 

 A. Shell permanently attached to the substratum (sea bottom, rocks, 

 other shells) either by direct cementation or anchored by a byssal 

 plug or by byssal threads. 



a. Attachment by direct cementation by the umbone or beak, the at- 

 tachment area producing a distinct scar. 



1. Attachment by the left valve which is generally the larger, the 

 shape usually irregular due to distortion, the valves alike or unlike 

 in surface sculpture. No teeth or other interlocking devices along 

 the hinge margin. 



Ostreidae 



2. Valves pectiniform, attached generally by the right valve, often 

 with a ribbed or spinous sculpture. Hinge provided with stout, 

 interlocking teeth or crura on each side of a deep, resilifer. 



Spondylidae 



b. Attachment by means of a solid byssal plug passing through a slit 

 or circular hole (foramen) in the beak and umbone of the right valve; 

 this opening sometimes closed off by later calcification, the shell 

 becoming free. 



3. Valves irregular, ostreiform, but with a subnacreous, platty or 

 micaceous texture, and a subtranslucent or silvery luster. 



Anomiidae 



c. Shell anchored by a byssus or a bundle of horny or silky threads 

 passing through a gap or notch along the anterior margin of the 

 right valve. Shell often aviculiform or with the ends unequally 

 winged or emarginated. Inner layer pearly. 



*A few small families arc not included in this key, as they are unimportant, their 

 characters are obscure so that their inclusion would make the key too complex for 

 general use. 



