MUSEUM OF COMPARATIVE ZOOLOGY. 435 



Deshayes's figure of a single lamella of Area Noa> would directly convey this 

 idea. But that is not the impression made upon me by a microscopical exam- 

 ination of the lamellae. They seem to me, in the specimens I have examined, 

 to consist of two rods, each with a chitinous skeleton or axis, united by a thin 

 transverse tissue, like the paper of a kite between the cords of its periphery. 

 An examination of the stages of development of the gill in some form of Pecten 

 or Area will be required to settle the true interpretation. 



In A rca ectocomata the stem of the gill is large, and the blood-vessels are 

 close to the inner wall as in Dimya, instead of being central as in Janira, 

 Amusium, or Perna. The edge of the stem is deeply grooved, and from the 

 opposite sides of this groove extend the gill filaments. On each side they 

 are placed in a single rank with a passageway between their bases, but they 

 curve toward one another at the lips of the groove, and nearly or quite touch. 

 In this manner a sort of half-closed passage extends between them. The 

 filaments, unlike those of the forms above described, are not supported by 

 chitinous rods; they seem to be simple tubes, abundantly ciliated, and in alco- 

 hol are contorted in all directions, twisted, and curled. The tips of the fila- 

 ments are slightly enlarged, but owing to their want of solidity and absence of 

 connection with one another, they appear not to form even as much of a sac 

 as in Amusium Dalli, unless the passage between their bases be so regarded. 

 (See Diagram C.) Taking for comparison A rca Noce, we find a proportionally 

 smaller stem, with a double series of chiunous rods on each side ; the pairs ol 

 each series are connected by delicate tissue only with each other, and these 

 combined groups correspond to the analogous series described in Janira. The 

 blood-vessel is central between their bases, which are separated externally by 

 an excavated space. The vein appears subtriangular in section. (See Dia- 

 gram D.) 



In Perna, Chione, and Cardium, the double series are pedunculated; in 

 Perna (Diagram H) the vein is central between the peduncles, in Cardium 

 it is double (Diagram G), one vein appearing at the base of eaeh sac; in Chione 

 it is much the same, but the outer branch of the vein is between the bases of 

 the outer primary lamella and the appendix or secondary lamella; the term 

 lamella in this instance being understood to mean a complete platoon, including 

 a double series of filaments. 



Whatever sort of nomenclature be used, it follows from the above interpre- 

 tation of the facts that the gills of Area ectocomata And Amusium Dalli are 

 just half the elements which go to make up the gills in Area Noa> and Janira 

 hemicyciica ; while in Dimya only one quarter as many elements are represented. 



In Nucula (Diagram I) we have a more specialized gill than in Dimya or 

 Amusium, though this will not be evident except by careful study. The gill 

 of Pleurolomaria, a much higher type of mollusk than Nucula, is more simple 

 than any of those above mentioned. It is composed of a cartilaginous adnate 

 stem, with lateral transversely striate lamellae (see Diagram F), a large and 

 a small blood-vessel. In section it appears much like that of Nucula or Sole- 

 nomya, but careful study shows the internal structure of the two to be differ- 



