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 AJ], 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, ete., if it corresponds to the first, second, or other 
segment formed by the folds of the lamella. Thus CA20 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 
Fie. 195. 
Sagittal median section of Teredo. a, anus; a.a, anterior adductor muscle ; a.s, anal siphon ; 
br.s, branchial siphon; c.g, cerebral ganglion; g, gill; h, heart; m, mouth; p.a, posterior 
adductor ; p.g, pedal ganglion; 7.0, renal opening; 7.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 lke 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, ¢.g., the dorsal sclerites of Pholas and the sclerites of 
certain species of Zhracia, 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 
