﻿P T E E V I) A. 



Genns HYALiEA. Lamarcl;. 



Testa hj/aUna, tenukmnui, subglobosa, lateralitei- appen- 

 diculata, symmetrica; apiee noiuipirali ; aperttira 

 anguda, laieraUter incisd ; lahro marginato, 

 pleriitnque in rostrum producto. 

 Shell glassy, very thin, subglobose, with lateral 

 appendages, symmetrical; apex non-spiral ; aper- 

 ture narrow, laterally incised ; outer lip bordered, 

 generally produced into a beak. 

 Synonym. Cavoliua Giceni. 



The Pteropoda are placed by authors, who have 

 studied their peculiarities, between the Cephalopoda 

 and the Gasteropoda. The name of the class is 

 happilj' chosen to suggest the wing-like pair of flap- 

 pers or fins which, placed on each side of the head, 

 effect by their action the locomotion of the mollusc. 

 The greater part of the species are common to all 

 seas ; a few being perhaps peculiar to the Atlantic, 

 and a few to the Pacific. They can be seen in the 

 evening or at night floating at the surface of the 

 water in great numbers, but at sunrise they sink to 

 various depths, and are seen no more till the luminary 

 of day disappears from the horizon. It is said that 

 each species has its own precise moment for retiring, 

 and its own appointed depth to which to sink. 



The shells of some of the genera are obliquely 

 spiral ; as Hemifnsus and Chelitropis. Some are 

 .symmetrically spiral. Some have au operculum. 

 Cymbulia, a boat-shaped symmetrical cartilage, but 

 no shell. The form of the shell in Hyalcea bears a 

 considerable resemblance to a symmetrical bivalve 

 shell with the valves soldered together at the dorsal 

 hinge. 



Species 1. (Mus. Brit.) 



Hyal^.4 nsciNATA. Hya. tesid teiiuissimd, siiccined, 

 tumidd, postice trispinosa, spinis lateralihus tri- 

 gonis, acutis, spina terminali deflexd, uncinatd ; 

 aperturd angustinsculd, lahro angtiste margiitnto, ! 

 medio paidi) jjrodncto. \ 



The hooked Htal^ia. Shell very thin, amber, | 



tumid, posteriorly three-spiiied, lateral spines 

 three-sided, sharp terminal spine bent down- 

 wards, curved ; aperture i-ather narrow, onter 

 lip narrowly bordered, a little produced in the 

 middle. 



Rang. D'Orbigny. Voy. Amev. Mcrid. PL V. f. 11 1.".. 



Hah. Atlantic Ocean. 



Distinguished from Hyala?a Trispinosa by the more 



pent-up form, and by the hooked character of the 



central spine. 



Species 2. (Mus. ? Copied.) 



Hyal.ea FLava. Eya. testdjiarid,!. triiiii^.iimd, ovatd, 

 subcompressd, postice trisj,i,ins,l, ,vy./«/s- luti'ralihns 

 suhapproximatis, spind cvulrnli rl,,iigiitd, afertiird 

 angustd, lahro hilohato. 



Thb; yellow Hyaljea. Shell flavid, very thin, oviil, 

 subcompressed, three-spined posteriorly ; lateral 

 spines rather approximate, central spine long ; 

 aperture narrow, outer lip bilobate. 



D'Orbigny. Voy. Amer. Merid. PI, V. f. 21 25. 



Ilab. Atlantic Ocean. 



Distinguished from Hyatea trispinosa by its oval 



and more compressed form. 



Species 3. (Mus. Brit.) 



Hyalj:a affinis. Hya. Jestd dmsc fused, qiiai,i II. 



tridentata aiigustiord, magis rotundd, rosfro apcr- 



turee producto, hilohato. 

 The related Hyalsa. Shell deep brown, narrower 



than Hyuhea tridentata, the rostrum of thr 



mouth produced, bilobate. 

 D'Orbigny. Voy. Amer. Merid. PL V. f. 6-10. 

 Rah. Atlantic Ocean. 



By the character of the shell alone the species 

 would scarcely be distinguishable from the common 

 species, and for this reason the writer has confined 

 himself to the above very short comparative descrip- 

 tion. 



