PECTIMD.fi. 



PECTIXIDjE. 



than ia UM jiaiiM pert of the molluscs of the same class. When it 

 ie M* contracted UM valve, are widely opened, and the animal has the 

 powvr of liiliiiili OB it frequent and sudden contractions, the 

 rapidity uf which facilitated by the extreme elasticity of the ligament 

 of UM valves. By means of these reiterated contractions the animal 

 caa nutter ia UM water, to UK> the happy expression of M. Quoy, and 

 one moet run after it to catch it among the corals or in the shallows 

 Where it dwells. M. Deshayes concludes that, from this remarkable 

 aba of characters, it might be possible for aoologists to decide upon 

 forming of tab genus a small family distinct from the Pectens, but 

 nlaoeTm their aeLhbourhood. 



The power of fluttering through the water is possessed by the 

 Pecesos [Coranruu.1 



The species of Lima an widely distributed, and is generally found 

 in the seas of warm and temperate climate*, where it has been taken 

 at depths ranging from the surface to thirty fathoms. 



Mr. Garner remarks that hi UM heart of Linta there are two 

 ventricles, the rectum passing between them. 



LtmatmJa (S. Wood) and Ltmtra (Brown) are sub-genera. Woodward 

 compute* the recent species at 20, and the fossil at 200. 



L. tjuumtif. Shell oval, depreessd, clipped as it were anteriorly, 

 white ; ribs sqnamoos, roach as a file ; hinge oblique, margin crenated. 

 It is a native of the seat of America. Then is a variety which has the 

 scales U*s numerous. 



The British species of Lima are L. nba*ric*lala, L. Lotcombii, and 



a- Animal oval, oblong ; the borders of the mantle dis- 

 united, thick, and furnished with many rows of tentacular cirrhi, 

 many of which are truncated, and terminated by a smooth and convex 

 surface. Mouth oval, furnished with great cut (dccoupcea) lips, and 

 oa eaoh aide with a pair of oblong and pointed labial palps. Branchiic 

 ia farm of a crescent, and formed of detached filaments. Foot rudi- 

 mentary, oo the disc of which a club-shaped pedicle raises itself. Anus 

 BoaUnr behind the adductor muscle of the valves. (Deshayes.) 



flbefl inequivalve, adherent, auriculated, beset with spines or 

 rough ; the umbooes unequal ; the lower valve offering an external 

 owdiaaj best which is flattened and divided by a furrow, and which 



T**e. with age. Hinge with two strotig teeth in each valve, and 

 an intermediate foseet for the ligament, communicating by its base 

 with UM external furrow. Ligament internal, the ancient remains of 

 which show themselves externally in the furrow. (Lamarck.) 



e* ; Meraal vkw of nlrea, sbevtaf hlofe, ligament, mtuctt- 

 *. , ppT rait* ; , lower alv. 



Thtsftaai with which the Sfondjti are armed, in some instances 



** ""* *raek everybody, and also that they bristle out 



wmthe upper valve. The lower valve is attached, and 



"^yy* b f U bT m *" M of foliations. If the whole 



* * n dose, not a spine 1* given out from 



pert of the fewer valve only, M is very frequently the case" 



etpecially when the shell is affixed among the branches of corals, a 

 favourite locality with some species, the foliations are confined merely 

 to that part where adhesion is required, and the rest, or free part of 

 the valve, is as profuse of spines as the upper valve itself. There 

 are two points to be gained, support or adhesion, and defence ; the 

 first is of primary importance, but as soon as that is safe, all the 

 resources of the animal seem to be turned towards its defensive and 

 offensive armour. Those fishes which browse among the corals are 

 thus deterred from iujuring the living fixture which has there taken 

 up its abode. A very fine series of specimens was collected with a 

 view to this habit, and they showed not only the power which the 

 animal had of secreting the proper process of shell according to the 

 circumstances required, but of modifying the secretion according to 

 the exigencies of the occasion. 



Spondyliu Americantu, with the va'.vca closed; the umbonra towards the 

 spectator. 



But there is another, and more interesting phenomenon, well dis- 

 played in one of the species of this genus. Professor Owen, having been 

 led to reflect, while considering the uses of the camerated part of the 

 shell of Nautiltu, upon the degree or extent of that structure as 

 possibly dependent upon the mode of growth of the animal and ita 

 shell, and how far it was a necessary physical "consequence of the 

 increase and change of position of the animal, independently of any 

 special purpose served by the forsaken parts or chambers of the shell, 

 had paid attention to all the cases that had come under his observa- 

 tion of the formation of chambers in shells, by the secretion, on the 

 part of the animal, of a nacreous layer, forming a new basis of support 

 to the soft parts, and cutting off tbo deserted portion of the shell 

 from the chamber of occupation. In a paper on the ' Structure of 

 the Water-Clam ' (Spondylut raritu, Brod.), read before a meeting of the 

 Zoological Society of London, he observes that it is well known that 

 the process above adverted to is not the only mode adopted to suit 

 the shell to the changing form and bulk, or other exigencies of its 

 occupant. Taking the genus MayUtu, for one example, the Professor 

 remarkn that the part of the shell from which the body gradually 

 recedes is filled up by the continuous compact accretion of calcareous 

 matter, and a solid massive elongated shell ia thus produced, which 

 would be a great incumberance to a locomotive mollusc, but ia of no 

 inconvenience to a univalve destined by nature to live buried in a mass 

 of lithophytous coral. 



Other instances were observed in Helix decottala and in the chambers 

 formed in Septaria. Among the bivalves the Ottrta not unfrequently 

 present shallow and irregular chambers in the substance of the shell, 

 and the f'thtriic have vesicular cavities interposed between the testa- 

 ceous latninm ; but the most constant and remarkable example of the 

 camerated structure of the shell is presented by the large Spondyliu, 

 or Water-Clam, above named, so called from the fluid which (until 

 lost by slow evaporation) occupies the chambers, and which is visible 

 in the last-formed chamber through the thin semi-transparent exposed 

 septum. 



In order to examine this camerated structure, and more especially 

 to see how it wai modified by the presence and progressive change of 

 place of the adductor muscle, he had a fine specimen sawn through 

 vertically and lengthwise ; it measured 8 inches in length ; the sub- 

 stance of the concave valve, which was 2J inches in thickness at the 

 thickest part, included 14 chambers separated from each other by very 

 regularly formed and stout partitions, composed, as in other chambered 

 shells, of the nacreous portion or constituent of the shell. The septa 

 were slightly undulating in their course, but presented a general con- 

 cavity towards the outlet of the shell. Not any of these partitions 

 were however continued freely across the shell, but each became con- 

 tinuous at the muscular impression, which is near the middle of the 

 shell, with the contiguous septa. In general also the septa commenced 

 singly from the cardinal or upper wall of the valve, and divided into 

 two when about one-fourth of the way towards the opposite or lower 

 wall, the thickness of the undivided part of the septum being equal 

 to, or greater than, that of the two divisions of layers into which it 

 splits. 



