358 



MOLLUSCOIDEA 



PHYLÜM V 



of lime. In Crania the shell substance is homogeneous, and punctured by 

 distally branching canals. In the Lingulidae and Obolidae the shell consists 



of alternating layers of phosphate of 



lime, admixed with lime carbonate, 



and a lustrous horny substance known 



as ceratin. The calcareous layers are 



prismatic, and are traversed by fine 



tubuli (Fig. 528). It 



is believed that the 



function of the punc- 



tae is for respiration, 



but the fact that these 



extensions of the 



mantle are not ex- 



verticai section posed to the water 



of a Lingula shell 



Fig. 527. 



A, Punctate exterior of a Terebratuloid shell, 

 slightly enlarged. B, Vertical section of shell of 

 Magellania flavescens, showing distally enlarged tubules, 

 Joo/j. c, Inner surface of Magellania shell, showing 

 ends of tubules and oblique calcareous prisms, loo/^ 

 (after Carpenter). 



sLrvinT anernate Hiay not aCCOrd with 



corneous («) and this explanation. 



calcareous (ö) layers. tt- f -r. 



Strongly magnified VolveS. BrachlO- 



(after Gratiolet). t i t i i 



pods are delicately 

 constituted animals, covered by two very vascular mantle lobes which secrete 

 the calcareous or corneo- calcareous valves, of which one is dorsal and the 

 other ventral in position. The valves are often thin and of unequal size, but 

 the inequality is rarely of such a nature as to disturb the bilateral symmetry 

 of the shell. 



During life the ventral valve, which is commonly the larger of the two, occu- 

 pies typically a superior position, and the dorsal is down. But in describing 

 the shell, it is always so oriented that the posterior margin, or hinge-line, is 

 placed above, and the anterior one below. A line drawn from the beak to the 

 anterior margin describes the length ; and one at right angles to the same, in 

 the direction of right and left, the width ; a third line drawn perpendicularly 

 to the other two, and passing through the centres of the valves, measures the 

 thicJcness. In the Protremata and Telotremata the ventral valve is convex, and 

 curved in such manner at the posterior margin as to form a heak The beak 

 may be pointed, or it may be perforated by a round opening, or foramen, for 

 the Protrusion of the pedicle. In many cases, however, the pedicle opening 

 lies underneath the apex of the beak, and sometimes encroaches upon a 

 portion of the dorsal valve. In the Atremata the pedicle emerges from 

 between the two valves ; in the Neotremata the posterior margin of the 

 ventral valve is notched, or there may be a small, circular, eccentric Per- 

 foration, or a more or less long, narrow slit. In the Telotremata the pedicle 

 opening, or delthyrium, which is originally triangulär in form, becomes either 

 wholly or partially closed by the growth of deltidial plates. In the Protremata 

 the delthyrium is closed by a true deltidium, while in the Neotremata and 

 Atremata a similar looking plate may be present, but as it is of a difFerent 

 origin, being secreted by the mantle, it is called the homceodeltidium. 



The Cardinal area is a term applied to the flattened or curved triangulär area 

 which is frequently observable between the hinge-line and the beak (Fig. 529). It 

 is more highly developed in the ventral than in the dorsal valve, and is bisected 

 medially by the triangulär delthyrium. A true cardinal area is absent in the 

 Atremata and Neotremata ; but when a small area is present in these Orders, it is 



