580 LECTURE XXIII. 



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 lodg- 

 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, subti'iedral 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 (^r), of a sub- 

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

 fewer cuts (yfig. 216, e), so as to present the appearance of a number of 

 close-set transverse plates, slightly indented by a median longitudinal 

 impression (ib. /"). This modification must increase the prehen- 

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

 is on the corresponding part of the larger and fewer tentacles of the 

 dibranchiate cephalopods that the acetabula are developed. The 

 angle between the two outer finely annulated surfivces subsides near 

 the end of the tentacle, which thus becomes flattened. 



To the nineteen tentacula which are supported by the confluent 

 and free digitations on each side of the head, two others must be 

 added, which project from very short sheaths, one before, the other 

 behind, the eye ; the lateral transverse incisions are deeper in these 

 than in the digital tentacles. The eyes are about the size of hazel 

 nuts, and are attached each by a short peduncle to the side of the 

 head, behind the digitations, and a little below the margin of the 

 hood. The inferior surface of the oral sheath is excavated for the 

 lodgment of the infundibulum. 



The mouth is armed Avith two mandibles, shaped, as in other 

 cephalopods, like the beak of a parrot reversed, the lower mandible 

 (yfig. 214, I) overlapping and curving upwards beyond the upper 

 one {K). Both mandibles are horny, with their tips encased by dense 

 calcareous matter, and their base implanted in the thick muscular 

 pai'ietes of the mouth. 



They are immediately surrounded by a circular fleshy lip with a 

 plicated anterior border, external to which there are four broad flat- 



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

 CCCLXXXVIII. 



