38 MOLLUSCA FROM THE CRAG. 



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 Invertebrata 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 Mamma- 

 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. /. Sow. Tab. IV, fig. 7, a — d. 



Purpura tetragona. J. Sow. Min. Conch, t. 414, fig. 1, 1823. 



MUREX ALVEOLATUS .... t. 4 1 1, fig. 2, 1823. 



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



Purpura tetragona. S. Wood. Catalogue 1842. 



P. Tesfd ovato-ventricosd, vel elongatd, subfusiformi ; transversim sulcata, sulcis late 

 profundi s, decussatis ; anfractibus super ne plana tis ; aperturd ovatd ; canali retiusculd ; 

 columella subperforatd ; labro intus deuticulato. 



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

 decussated, 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, \\ 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 alveolatus, 

 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 convoluted cone, as the markings are precisely similar, and the 

 aperture of the same form. It has four or five very elevated and rounded trans- 

 verse ridges, with one, or sometimes 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 d 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 in the var. tenerum of Buccinum undatum. 



