GENUS POLYMORPHINA. 1G7 



SerpuJa, witli the description tenuis ovalis Icevis, is given to the well-known form afterwards 

 named by Walker -S*. ladea ; under which designation it has been cited by several among the 

 earlier writers on British Conch ology. Certain aberrant forms distinguished by the 'stag- 

 horn' processes to be presently described, were described and figured by Soldaui (ci) under 

 the name Pohjmorphia corcula spinosa ; and upon his figures of this and certain other forms 

 of the same type, De Montfort (lxvii) founded his genera Canopus, Cantharus, Misilus, and 

 Areihusa, of which none but the last has been adopted by any other systematists, and of which 

 all are now entirely disused. For the objectionable name Serpula, the scarcely less objectionable 

 name Vermicidum was substituted by Montagu (lxv), who associated under that generic desig- 

 nation examples of Miliola, Lagena, and Polpiioiyhina ; and this, again, has been adopted by 

 British Conchologists, though it has not been admitted by continental authorities. The name 

 Polgmorjj/dna was used by D'Orbigny in 1825 to designate a portion of tiie series of forms 

 which we now include under that generic type; other portions being separately ranked by 

 him under the genera Globulina and Gidlidina, which, together with Dimorphina, made up his 

 family Poli/morpInnidcB. The absence of any valid generic or even specific distinction between 

 the ordinary forms of Polijmorpldiia, Globulina, and Gidtidina became apparent to Prof. 

 Williamson from the 'comparison of only the British species which had fallen under his special 

 observation ; and we can not only entirely endorse his excellent remarks (ex, p. 73,) upon the 

 specific identity of the forms whicli he brings together under tlie designation Polymor- 

 pldna ladea, but can extend them also to those other more divergent modifications of which 

 his P. mijristiformis is an example. 



20 1. External Cliaraders and Internal Struciure. — The essential characters of Polijmor- 

 pJdna consists in the biserial alternation of lageniform chambers, each with a i-adiating mouth, 

 along the two sides of a longitudinal axis. The form and disposition of the chambers, 

 however, may vary so much as to give to the entii'e shell such marked diversities in shape, 

 that, unless tiie intermediate gradations were traced out, there would seem adequate ground 

 for specific if not for generic differentiation. Starting from what may be regarded the typical 

 form of the common P. ladea (ex, fig. 145), we observe that the segments are arranged very 

 obliquely to the central axis, and that each is prolonged in some degree over tlie penultimate 

 segment on the opposite side (so as to render the shell unsymmetrical), whilst it also extends 

 itself backwards over the antepenultimate segment on its own side ; the entire shell thus 

 presenting a somewhat fusiform shape. Now there may, on the one hand, be a greater elongation 

 of the shell, arising out of a predominance of the forward over the backward g>owtli of the seg- 

 ments, so as to make the entire shell resemble a wheat-ear or grass-spike ; whilst, on the other 

 hand, by a predominance of the backward growth of the newer segments, the earlier ones may be 

 almost or completely concealed by them, and the entire shell may come to present the form of 

 a drop or a tear (ex, figs. 147, 153 — 155). Again, its two opposite surfaces may be more or 

 less compressed ; and this compression may proceed to such an extent that tlie proportions of 

 the shell resemble those of a thick fleshy leaf. The most remarkable departure from the 

 ordinary type is seen in those forms in which the last chamber extends itself over all the 

 preceding, so as completely to include them ; this is especially the case in some forms, which, 

 from tjicir peculiar mode of growth, appear to have been adherent (ex, figs. 151, 152). The 

 outermost segment may develope itself into numerous irregular expansions and tubular growths, 



