REPORT OX THE PTEROPOHA. 
85 
the roof (superior or dorsal wall) of the buccal cavity of an armature of 
rather strong and somewhat recurved hooks (PL IV. fig. 5). 1 I have satis- 
fied myself of the existence of this armature in Notarchus punctatus and 
Dolabella neapolitana . 2 
On comparing this armature, not with the hook-sacs of Pneumono- 
clerma, which present the most highly specialised form of these organs, but 
rather with those of Dexiobranchsea (PL IV. fig. 6), which actually present 
the primitive condition, it is easy to understand that it is not a great step 
from the dorsal buccal wall, covered with hooks, to the small depressions 
filled with hooks found in Dexiobranchsea. 
What, however, is the morphological value of this armature of hooks in 
Notarchus and Dolabella and of the hook-sacs of the Gymnosomata ? 
The comparative study of the horny buccal pieces other than the radula 
is attended with peculiar difficulties, for these pieces have never been the 
subjects of systematic investigation as has the radula. The knowledge of 
these organs which we possess is then very fragmentary, and there are 
scarcely any synthetic documents relating to them. 
The only attempt to treat these pieces systematically was made by 
Mdrch more than twenty years ago. 3 But this author, besides including 
among the extra -radular pieces organs which have no place there [e.g., the 
pickaxe-like organs of Conus, which correspond morphologically to the 
radula), multiplies the number of different species which, according to 
him, may be distinguished among these organs, so that he by no means 
facilitates the systematisation and homologisation of these extra-radular 
pieces. 
These horny extra-radular pieces are wanting in certain Gastropod 
Mollusca — Toxoglossa [Conus, Pleurotoma, &c.), Pyramidellidse, Eulimidse, 
many Trochidse, Heteropoda, Gadinia, Amphibola, Testacellidse, many 
Nudibranchs [Doridopsis, Hermsea, Tethys), Phyllidia, Pellibranchia, 
certain Tectibranchs [Actseon, Utricidus, Scaphander, Doridium, Lobiger). 
But the great majority of the Mollusca called “ Oclontophora ” possess 
horny buccal pieces other than the radula. 
As regards the relative position of these organs, we may state in the 
first place that the radula is always situated posteriorly to every other horny 
1 Ibid., pi. iv. fig. 90. 
- Dolabella neapolitana, delle Chiaje, seems to me to be identical with Aplysia petalif era, Rang, Aplijsia webbii, van 
Beneden, Aplysia depressa, Cantraine, and Dollabella ornata, Deshayes; on the other hand, however, it appears to 
constitute a genus ( Aplysiella ) distinct from Aplysia and Dolabella. It will probably be necessary therefore to call 
this species Aplysiella petalifera (Rang). 
3 On the Homology of the Buccal Parts of the Mollusca, Ann. and Mag. Nat. Hist., ser. 3, vol. xvi. p. 73. 
