PULLASTEA. 119 



open, having the anteal and posteal portions of the margin 

 dentated, the middle is smooth and gently sinuated. The 

 siphons, when extended, are fully as long as the shell, and 

 united to about ^ of an inch from their extremities, when 

 they become well separated, the anal one curving upwards, 

 the branchial inclining downwards ; the former is circled with 

 20-30 pale brown short cilia, sometimes tipped with white, 

 the latter with a variable number of longer brown or pale red 

 ones, each being fringed on both sides with short horizontal 

 white fimbrise; there are also 16-30 shorter intermediates, 

 some plain white, some brown and fimbriated on both margins; 

 the siphons are white, except a short area at their termi- 

 nations below the cilia, of red-brown dark bistre, or of those 

 colours mixed, and marked with short fine red transverse 

 lines and longitudinal blotched ones. A pair of pale brown 

 branchise on each side; the upper laminse not covering the 

 lower, decussated on both surfaces by the network of the 

 blood-vessels; the palpi are small, triangular, striated on the 

 inner surface, and smooth without ; the posterior ends of the 

 branchiae are permanently fixed in this species and the P. de- 

 cussata to the roof of the branchial tube, as in Pholas. 



The Venus perforans of authors is only a dirty -white or 

 ochraceous specimen of this species, arising from the exclusion 

 of light, in consequence of its habitat being adventitiously cast 

 on rocks, where, like the Saxicavce and other excavators, it 

 has the power of imbedding itself. We have carefully ex- 

 amined and compared with them many of the free shells from 

 the shingles, and have not detected the slightest specific 

 difference : we consider the two identical. The foot of this 

 species is pure milk-white, with a byssal groove producing 

 fine filaments, which, when the animal is not in pure sand, 

 attach it to the shingle ; it is muscular, slightly geniculated, 

 and lanceolate. 



This is strictly a littoral species, never being met with in 

 the dredge, and is scarce on the shingle tracts at Exmouth. 



The variety, the V. perforans of authors, is abundant, being 

 imbedded with the Pholas dactylus in the red sandstone at 

 the same place. 



