272 GILBERT C. BOURNK. 



opening into it represents the pericardial cavity, nor can 

 there be any doubt that the branching canals are homologous 

 with the structures described by Sassi in Anoniia ephip- 

 piuni. If the lumen represents the pericardial cavity, the 

 glandular tissue clearly is the homologue of the pericardial 

 gland, and the chief point of difference between Enigma 

 and Anomia is that in the former the pericardial gland is 

 well developed, and retains not only its function as an ex- 

 cretory organ, but also its connection with the heart. As is 

 shown in figs. 4 and 14, and in text-figure 2 a and b, I. pg., 

 the left pericardial gland is of considerable vertical extent in 

 the immediate neighbourhood of the left reno-pericardial 

 canal. Posteriorly it narrows to a thin band of glandular 

 tissue, which may be traced backwards, lying to the outside 

 of and above the upper limb of the left renal sac, and in 

 close connection with the anterior of the tAvo veins into which 

 the left auricle divides, as far as the muscular wall of the 

 left auricle itself. As it approaches the heart the peri- 

 cardial gland spreads out over the walls of the auricle and 

 becomes intimately fused with them, extending, as 1 shall 

 show further on, into the thickened walls of the ventricle. 



On the right side the right reno-pericardial canal (figs. 4 

 and 11, r. ly.) leads into the lumen of a similar but larger 

 mass of glandular tissue, occupying the right wall and floor 

 of the dilatation of the right auricle described on p. 260. 

 This dilatation of the blood- vessel has no apparent analogue 

 in Anomia. The glandular sac forming part of the thick- 

 ness of its walls is of course the right pericardial gland, and 

 its histological structure as well as its relations to the upper 

 posterior lobe of the right renal sac and to the right auricle 

 are analogous to those of the left pericardial gland. The 

 extent of the two glands and their relations to the kidneys 

 and heart are indicated by the spaces enclosed between the 

 black lines in fig. 4. 



The structure and distiibution of the pericardial glands of 

 the Lamellibranchia has been worked out in great detail by 

 Grobben (6). Without entering into the details of his careful 



