CEMENTING APPARATUS. 145 



not able to trace these ducts to their glands. In these 

 zig-zag ducts, and in the rudimentary points sometimes 

 observed at the base of the duct (c), and likewise at the 

 base of the spur (b), we see the first indication of that 

 tendency to bifurcation, so strongly characteristic of the 

 cement-ducts in all the genera, excepting those already 

 described, which are allied to Coronula. 



Chelonobia patula. — The cementing apparatus is here 

 chiefly remarkable for the thinness and straightness of the 

 main trunk, (//, PL 28, fig. 2), and from the great distance 

 at which the glands stand apart ; had another gland been 

 drawn, it would, on the scale here used, have stood exactly 

 under the two upper, [d c') in fig. 1 c. We here see that 

 the trunk (/), before entering the gland (h), has an en- 

 larged portion (^) ; this, I suspect, is a very general struc- 

 ture. Each gland gives out, on opposite sides, two ducts 

 {a a,bb)> larger even than the main trunk ; and these ducts 

 bifurcate repeatedly, and inosculate. By this inosculation it 

 is not improbable that all four ducts, proceeding from the 

 two glands of the same age, may be connected together; 

 certainly the bifurcating branches from the same duct thus 

 become repeatedly connected. For the first two or three 

 bifurcations the ducts decrease very little or not at all in 

 diameter ; but nearer the circumference they become 

 smaller. The ducts, also, proceeding from the younger 

 and smaller glands, are, of course, proportion ably smaller. 

 In one case I was able to count four bifurcations in the 

 duct between the gland and the edge of the basal mem- 

 brane. It follows from this structure, that the basal mem- 

 brane, at each period of growth, is cemented down by cement 

 issuing from several orifices ; but we shall presently find 

 that in other genera the cement proceeds from many more 

 orifices. In fig. 2 there is represented, by the aid of the 

 camera, a small portion (from the outer (a) to the outer (b) 

 being ^ths of an inch in length) of the basal membrane, 

 with all the several cement-ducts adhering to it, which I 

 could distinguish, and drawn of their proper relative sizes ; 

 this figure also shows some of the bifurcations, but no inos- 

 culation happened to be included in the space here given ; the 

 basal membrane itself has not been represented. In taking 



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