CAECUM. 323 



exotic Vermetus, both as regards animal and shell, cannot be 

 placed satisfactorily in any of the groups between the Helicida 

 and Pyramidellidte. The reasons for this arrangement are so 

 fully explained in the subjoined prologue on C&cum, as to 

 require no further immediate comment; and those relating 

 to Twrritella will also be found in our notes on that genus. 



CAECUM, Fleming. 



This singular genus has long been the reproach of concho- 

 logists, and the pons asinorum of this branch of science ; 

 the greatest names have failed to see its true position ; it has 

 run the gauntlet from Dentalium, &c. of the Mollusca, to 

 the Foraminifer Orthoceras. We do not attribute the satis- 

 factory solution of the problem to any hypothesis of ours, but 

 to time and opportunity, which, almost against hope, pre- 

 sented the living objects of our research. The animal, in 

 essentials, whether it be considered an aberrant Vermetus, 

 Siliquaria, or Caecum, must in all probability follow Trochus, 

 with Turritella, as I think these genera will prove herm- 

 aphrodites. 



At one time we were almost inclined to substitute but 

 our reverence for priority prevailed Vermetus for Caecum , in 

 consequence of the animal and shell of the two being so 

 much alike. The shell of C. glabrum, when very young, like 

 Vermetus, forms spiral turns before it commences the elonga- 

 tion ; these in Caecum, as well as many of the subsequent 

 portions, as the animal grows and withdraws itself therefrom, 

 fall off, the cylinder above them having been previously 

 plugged up ; Vermetus does the same, but remains with the 

 terminal spiral attachment. I also believe that the other 

 species, the C. trachea, has a right to claim the Skenea rota of 

 authors, or an object closely similar, for its spiral turns, in a 

 very young state ; in it the three or four epochs of growth are 

 as distinctly visible as in the elongations of the cylinder of 

 the C. trachea at maturer ages. Also, under the microscope, 

 the apex of the young C. glabrum, the Serpula incurvata of 

 Montagu and Walker, agrees with that of the so-called 



Y2 



