38 MOLLUSCA FROM THE CRAG. 



the generality of recent specimens, either from the British seas or from those of 

 America, and also to have exhibited a greater range in variation ; although Dr. Gould 

 remarks, in his very able descriptions of the Invcrtebrata of Massachusetts, that scarcely 

 two specimens can be found alike on that shore, where it is abundant. It is quite a 

 littoral species, and is found upon rocks up to high-water mark. In the Marama- 

 liferous Crag it is associated with estuary species. It first appears in England in the 

 Red Crag, which seems to have been peculiarly favorable to its development ; and, as 

 Professor E. Forbes suggests, in his ' Report upon the Geological Relations of the 

 Existing Fauna and Flora of the British Isles,' p. 93, is probably of American origin. 

 Mr. Lyell speaks of it as a fossil of that country, in his paper upon the Miocene Tertiary 

 Strata of Maryland, &c. 



2. Purpura tetragona. J. Sow. Tab. IV, fig. 7, a — d. 



Purpura teteagona. /. Sow. Min. Conch, t. 414, fig. 1, 1823. 



MUEEX ALVEOLATUS. .... t. 411, fig. 2, 1823. 



— Nyst. Coq. foss. de Belg. p. 547, pi. 43, fig. 1, 1844. 



PuKPUKA TETRAGONA. iS. TFood. Catalogue 1842. 



P. Testa ovato-ventricosd, vel elongatd, mhfusiformi ; transversim sulcata, sulcis late 

 profundis, decussatis ; anfractibus sn/perne planatis ; aperturd ovatd ; canali rectiusculd ; 

 columelld subperforatd ; lahro intus denticidato. 



Shell ovate, ventricose, sometimes subfusiform, and elongate ; sulcated and decus- 

 sated, forming large and deep alveoli upon the exterior ; volutions rather flattened 

 above, subcarinated ; aperture ovate, with a short and open canal, a little inclined 

 backwards ; columella subperforated ; outer lip dentated within. 



Axis, li inch. 



Locality. Red Crag, Sutton and Walton Naze. 



This shell appears to have had a very limited vertical range as far as it is at present 

 known. I have met with it only in the Red Crag, where it is exceedingly abundant, 

 and very variable in its proportionate dimensions. Murex akeolatus, Sowerby, is, I 

 imagine, only an elongated variety of this species, as the two may be connected by 

 every intermediate form between the extremes figured (fig. 7, a — b), by the suite of 

 specimens in my own cabinet. The only difference is, a greater prolongation of the con- 

 voluted cone, as the markings are precisely similar, and the aperture of the same form. 

 It has four or five very elevated and rounded transverse ridges, with one, or some- 

 times three, intermediate striae ; these are decussated by the raised and subfimbriated 

 edge of the outer lip, thereby producing upon the exterior the deep alveoli which so 

 greatly distinguish this shell. The left lip is thin, with the columella a little flattened ; 

 and in the adult shell the outer lip has about half a dozen denticulations on the inside ; 

 the flexure of the canal leaves the umbilicus partly open. Fig. 7 6? is, I presume, a 

 variety of this species, which has lost a portion of its outer coating, and is thinner, 

 much after the manner in which the substance of the shell is reduced m the var. tenerum 

 of Buccinum undatum. 



