the Buccal Parts of the Mollusca. 75 



Proctonotus mucroniferus Alder and Hancock have discovered a 

 " corneous transverse arch or strap" uniting the two halves, and 

 reminding us of the middle maxilla in the Pulmonata. The 

 nature of the mandibles described by Troschel (Gebiss der 

 Schnecken, t. iii. f. 18) in Pneumodermon is, in my opinion, very 

 doubtful. 



The cheek-plates*, or immoveable mandibles, are two lateral 

 plates, without cutting edges, composed of scaly or acicular f 

 particles, which seem only of use to protect the inside of the 

 mouth from injury by the spinous tongue. Sometimes the two 

 plates are united above by a ligament. This kind of mandible is 

 found in nearly all the Tcenioglossata, as well in those provided 

 with a rostruin {Cyclostomus, Valvata, Rissoa, Jeffreysia, Crepu 

 dula, Vermetus, Trichotropis, &c.) as in those with an haustellum 

 {Marsenia, Natica, Cypraa, Cassis, Triton, Strombus, &c.) : vide 

 Troschel, Gebiss derSchnecken. These plates are apparently want- 

 ing in all the Bhachiglossata {Murex, Fusus, Nassa, &c.). The 

 linear horny plates described in Buccinum undatum by Cuvier 

 (Anat. des Mollusq. figs. 11, 12) and by Valenciennes (Archives 

 du Museum, t. v. p. , t. 25. f. 6) are probably appendages of 

 the tongue, and used as a handle in perforating the shells on 

 which they prey. 



The prehensile collar (Hancock) is a circular band composed 

 of conical, often forked, erectile spines. Sometimes it is entire, 

 as in Ancula cristata and Goniodoris nodosa; in others it is 

 divided into two lateral parts, not unlike cheek-shields, as in 

 Idalia pulchella and Acanthodoris pilosa. 



Evertile cheek-cushions (" lames lat^rales," Lacaze-DuthiersJ), 

 two large evertile lateral sacs, covered with close-set lanceolate 

 plates, like pavement, in Pleuroh^anchus. They seem to be an 

 intermediate form between the prehensile collar and the evertile 

 arms of the Gymnosomata and Cephalopoda. 



Cheek-hooks or cheek-crooks (harpagse§); mandibles, Eschricht, 

 in Clione; "les caecums de la bouche, avec un tube corne dans 

 Pinterieur" (Van Beneden||); "evertile tubes serving as pre- 

 hensile organs" (Troschel^) (this author has not detected these 

 organs iu Clione and Pneumodermon)-, " lateral or cheek-pouches" 

 (Macdonald**. Eschrichtft fii'st showed that these tentacle- 



* "Mandible-like parts, indumentum epitheliae, the horn sheaths of 

 the inner lip" (Bergh). 



t In Thecosomata [Hyalcea, &c.) they are composed of four or five 

 band-like plates {vide Troschel, Gebiss). 



X Ann. des So. nat. ser. 4. vol. xi. t. 7. f. 4. 



§ 1 intentionally use this form of the word. 



II Exercices zootomiques, ^ Gebiss d. Schn. 



•* hoc. cit. and Linn. Trans, xxii. 1858, p. 248. 



tt Anatomiske Undersbgelser over Clione borealis, 1838. 



6* 



