214 



THE LAMELLIBRANCHIA 



Sundry methods have been proposed for representing the hinges 

 of Lamellibranchs by formulae. The most logical is that of Munier- 

 Chalmas and Bernard, which takes the origin of each element into 

 account. In this system the primitive lamellae are indicated by 

 Roman numerals, even numbers being used for the left valve, odd 

 numbers for the right valve, preceded by the letter A if they are 

 anterior to the ligament, by the letter P if they are posterior to it. 

 Each definitive tooth is indicated by an Arabic numeral corresponding 

 to the number of the primitive lamella from which it is derived 

 (A2 in the case of All, etc.), and is preceded by the letter C if it 

 is a cardinal, or L if it is a lateral tooth, and is further followed by 

 the letters a, b, etc., if it corresponds to the first, second, or other 

 segment formed by the folds of the lamella. Thus CA2& stands 

 for the left cardinal tooth originating from the posterior segment of 

 the anterior lamella II. 



In some exceptional cases the two valves of the shell are fused 



FIG. 195. 



Sagittal median section of Teredo, a, anus ; a.a, anterior adductor muscle ; a.s, anal siphon ; 

 6r.s, branchial siphon ; c.g, cerebral ganglion ; g, gill ; h, heart ; m, mouth ; p.a, posterior 

 adductor ; p.g, pedal ganglion ; r.o, renal opening ; r.p, reno-pericardial orifice ; v.g, visceral 

 ganglion. (Partly after Grobben and Beuck.) 



together dorsally ; such shells are called symphinote, and examples 

 may be found in the adults of some species of Pinna, Unio, Anodonta, 

 and Hyria. But even when the borders of the mantle are almost 

 completely fused together on the ventral side, the two valves are 

 never fused ventrally to form a single tube like the shell of the 

 Scaphopoda. Nevertheless, when the shell gapes and the mantle 

 borders are largely fused together and provided with long conjoint 

 siphons, the portions of the mantle that project beyond the valves 

 may secrete a long calcareous tube (Teredo, Fistulana), which may 

 be fused to the shell, as in Aspergillum, or the two valves themselves 

 may be fused together dorsally as in the four genera mentioned 

 above. In some other siphonate Lamellibranchs with gaping shells 

 the portions of the mantle that project from the shell may secrete 

 accessory protective sclerites, which may be independent of the 

 valves, as, e.g., the dorsal sclerites of Pholas and the sclerites of 

 certain species of Thrada, or may be united to the shell as is the 

 siphonal tube of Pholadidea. In Teredo two movable calcareous 

 plates, actuated by special muscles, are formed symmetrically right 



