BIVALVIA. 49 



as the animal decayed, though the valves are not precisely adherent as they are in life. 

 But for this I should have supposed it to have been a derivative from the Coralline Crag, 

 from which the smaller specimen shown in fig. 7 of Tab. VI was obtained. I at first 

 thought that it might be the same as the Pkolas brevis from the Cor. Crag, of which I 

 was enabled to figure a fragment in my first Sup. (Tab. X, fig. 24) ; but the differences 

 are so great that I cannot regard the two as identical. Both shells, however, belong to 

 the true genus P kolas, and not to that section of it called Zirpkea, which was proposed 

 as a separate genus by the late Dr. J. E. Gray ; and in which the rays are confined to 

 the anterior portion of the shell, and are bounded by a deep sulcus ; and to which 

 section P. crispafa belongs. 



The specimen, consisting of a single and smaller valve, which is represented in Tab. VI, 

 fig. 7, was sent to me by Dr. Reed, with the name of Pholas parva attached, as from 

 the Coralline Crag of Gedgrave, but it seems so closely to resemble the large shell from 

 the Red Crag, represented in Tab. V, fig. 2, that I think it must be the younger state of 

 it. It differs bom. parva in being considerably shorter in proportion to its breadth, the 

 figure of that species from the Red Crag, given in the first Sup., Tab. X, being taken 

 from a specimen which had been somewhat distorted by confinement in the crypt, and I 

 have not seen that species in the Coralline Crag. I think it possible that the small 

 specimen represented in fig. 243 of Tab. X of my first Supplement, may be a still 

 younger state of our present shell instead of, as supposed in that Supplement, the young 

 of the shell represented in fig. 24a of the same plate (and which I retain as Pkolas brevis), 

 as it has a similar deep opening for the foot ; but a good series is required for a satis- 

 factory determination of that question. 



Pkolas dactylus, Linn., has been given as a species from the Red Crag in Mr. 

 Prestwich's paper ' Quart. Jour. Geol. Soc./ vol. xxvii, p. 485, and by Mr. Bell in his 

 paper on the English Crags, ' Proc. Geol. Assoc.,' vol. ii, No. 5, p. 26, from the "Middle 

 (or Oldest Red) Crag." I have procured from Dr. Reed the specimen upon which this 

 identification was based, and which has the locality of Walton Naze marked upon it, 

 and to set the subject at rest I have had it represented in fig. 5 of Tab. V. The 

 specimen exhibits unequivocally those characteristics which I have pointed out at p. 295 

 of the second volume of the ' Crag Mollusca ' as distinguishing cylindrica from dactylus, 

 and there can be no question of its being the common Walton species, Pit. cylindrica, 

 J. Sow. In the list 1 given in the lately published memoir of the ' Geol. Survey,' for half 

 sheet No. 48, this species is introduced, but this is probably only by adoption from the 



1 There are some errors in this list, even as regards Walton ; but that part of it which refers to 

 Beaumont (and which I presume is merely a repetition of the late Mr. John Brown's list of shells 

 obtained from that locality) is, in my opinion, quite untrustworthy. Pyrula uniplicata, Duj., given in the 

 memoir list, is probably a clerical error for some other shell, possibly PyramideUa unisulcata, which in 

 Mr. Prestwich's Coralline Crag list is regarded as identical with P. Iceviuscula, but which I c'o not 

 consider to exist in any part of the Crag. There is also a clerical error in respect of that shell in Mr. 

 Prestwich's Bed Crag list. 



7 



