CEMENTING APPARATUS. 139 



circumferential slip, to which the last-form eel pair of glands 

 and cement-ducts belong. In correspondence with the 

 great number and narrowness of the central slips of mem- 

 brane, so are the cement-glands towards the centre nume- 

 rous and very small. All the glands, in the more central 

 parts, consist of a mere transverse enlargement of the 

 cement-trunk ; but the exterior and larger glands, which 

 are more closely packed together, are more globular or 

 pear-shaped ; and the two ducts (of which the one on the 

 rostral side is considerably enlarged at its base) do not 

 come out of the gland exactly at the same level. The 

 trunk, connecting the glands, runs straight from one to the 

 other. The ducts proceeding from the outer and older 

 glands, on the carinal side, are much curved (PL 28, fig. 1 c). 

 To give an idea of the dimensions of the several parts, I 

 may state that the largest ducts were ^ths of an inch in 

 diameter, and the glands belonging to them nearly thrice 

 as much, measured in the direction of the cement-trunk ; on 

 the other hand, some of the ducts from the small central glands 

 had a diameter eighteen times less than that of the largest 

 ducts. Towards the circumference, the ducts that proceed 

 from the older and larger glands are piled one exactly over 

 the other — the last formed being the topmost, and all are 

 imbedded in the corium which overlies the basal membrane : 

 this position of the ducts, one over the other (which could 

 not be well represented in figs. I a and 1 c), is owing to 

 their all debouching at the same exact point. But the 

 ducts form the smaller and younger glands, when the shell 

 had a different shape, are spread out, and are all attached 

 to the basal membrane. Altogether, the basal membrane 

 of Coronula, when well cleaned, and examined under a 

 moderately high power, presents the most singular and 

 elegant appearance. 



We now come to the cement-tissue : this lies on the 

 under or outer side of the basal membrane; it is not repre- 

 sented in figs. 1 a or 1 c, but only in the sectional dia- 

 gram, 1 b, by the letters z, z : it presents its usual character 

 and appearance, like solid glue or brown gum, but is 

 obliquely laminated and sub-laminated : it forms a layer, 

 much thicker than the basal membrane itself, being as 



