2IO AN INTRODUCTION TO THE STUDY OF FOSSILS 



of these four gills is double, being a very narrow bag open dor- 

 sally. From the union of these gills with each other and with 

 the mantle on each side, and anteriorly with the viscero-pedal 

 mass (the main body mass into which the foot merges), it results 



Fig. 89, — Vertical section through the quahog, Venus mercenaria Linne. A, 

 looking into the shell toward the anterior end. a.a., anterior adductor muscle; 

 a.r., anterior retractor muscle scar; /., ligament showing its C-spring shape; 

 It., left valve; rt., right valve; s., section of shell showing the increments of 

 growth. B, an ideal-section through a pelecypod. Such a section may be corn- 

 pared to a book, with binding along the dorsal (ligamental) edge and with the 

 valves corresponding to the covers, the mantle lobes to the fly leaves, the two 

 pairs of gills to the first two and the last two pages and the foot to the remainder 

 of the leaves fastened together. 



that the gills form a partition extending through the mantle 

 cavity, dividing it thus into two chambers. Of these the upper 

 is called the cloacal and from it passes the exhalant or anal 

 siphon ; the lower is called the branchial and into it opens 

 the inhalant or branchial siphon (Fig. 90). 



The gills are vertically striated. Microscopic examination 

 shows these striae to be parallel hollow tubes. Thus it is seen 

 that each lamellar half of a gill is composed of a series of these 



