316 LECTURE XXIII. 



wards, forming a wide concave fold in the dorsal aspect (e), which is 

 reflected over the black-stained involuted convexity of the shell. The 

 margin or collar of the mantle is continued downwards and forwards 

 on each side with a sinuous outline, and is perforated below for the 

 passage of the muscular expiratory and excretory tube called the 

 funnel (i). Besides the muscularity of the free border of the mantle, 

 which indicates its power of extension and contraction, its surface is 

 studded with the orifices of many minute glandular crypts ; and it is 

 the organ by which the growth of the shell is principally effected. 

 The nidamental glands form two circular convexities on the ventral 

 surface of the abdomen, behind which the mantle is encircled by a 

 thin layer of brown matter, like the periostracum, which is very nar- 

 row above and below, but expands on each side into a broad plate, 

 corresponding in size and form with the surfaces of attachment of the 

 two great muscles for adhesion to the shell. 



The anterior or muscular division of the Nautilus, which may be 

 termed the head, forms a strong and wide sheath, containing the 

 mouth and its more immediate appendages ; its inner surface is for 

 the most part smooth, the outer one divided and extended into many 

 parts or processes. The chief of these forms a broad triangular mus- 

 cular plate or hood (/), covering the upper part of the head, and 

 presenting a middle and two lateral superficies ; the former being 

 traversed by a median longitudinal furrow, indicating the place of 

 confluence of the two large hollow tentaculiferous processes of which it 

 is composed. The back part of the hood is excavated for the lodge- 

 ment of the involuted convexity of the shell, and the above- described 

 fold of the mantle covering it. Each side of the head supports a 

 group of perforated processes or digitations, the largest of which is 

 next the hood, and the rest decrease in size as they descend in posi- 

 tion. Exclusive of the short subocular_, perforated process *, the digi- 

 tations are eighteen in number on each side disposed irregularly, but 

 all directed forwards, some not reaching as far as the anterior margin 

 of the head, others projecting a few lines beyond it. They are of a 

 conical, subtriedral form : the large one next the confluent pair which 

 forms the hood, has, like that part, a papillose outer surface. Each 

 process contains a long and finely annulated tentacle (g), of a sub- 

 triedral form, with the inner surface incised, as it were, by deeper and 

 fewer cuts {^fy- 132. e), so as to present the appearance of a number of 

 close set transverse plates, slightly indented by a median longitudinal 

 impression (Jiff- 132./). This modification must increase the prehen- 

 sile and sentient properties of the inner surface of the tentacle, and it is 



* Particularly described and shown not to be tentaculiferous by M.Valenciennes, 

 Nouvelles Recherches sur le Nautile Flambe, Archives du Museum, 4to. 1839. 



