f CALICULATION, CRESTS, ETC., IN BIVALVES. 43 
i 
I bear great resemblance to the dextral shells of some gastropods, the 
V right valve showing similar resemblance to sinistral forms. 
Caliculation or Capping of the Um- 
bones is apt to take place to a noticeable 
extent only in those .species in which the 
embryonal shell is of a comparatively large 
size and .somewhat globular shape and the 
succeeding shell-growth does not continue 
on the same plane, as is occasionally .seen in 
some species of Pisidhun and S2)ha’rium. 
The Posterior and Anterior Cre.sts are most remarkably 
developed and most noticeable in immature .shells, and mark olf the 
posterior and anterior liuuts respectively, of the upper or dorsal 
margin ; the posterior crest is sometimes distinctly and strikingly 
Fig. 106. — A “Crested” Bivalve. 
Anodonta anatina var. radiata Jeffr., right valve, 
River Foss, Blue Bridge, near York, collected by Rev. W. C. Hey, M.A. 
Showing the anterior and posterior crests of the dorsal margin. 
ax, anterior crest ; px. posterior crest. 
angulated in the adult shell of Anodonta, but in Unio the anterior 
crest is often the most strongly marked. 
The Rostrum or Beak is the produced posterior end of bivalve 
sliells, and its extent is sometimes clearly defined on the posterior 
margin by two bluntly angular ridges— the most ventral one being 
the gonial ridge — which run towards the umbones. The term beaks 
has often been applied to the umbones, but .should be discontinued 
to avoid the po.ssibility of confusion. Dr. Brot who has esiiecially 
Spiuerium /acustre (Miill.), 
right valve, X 3, 
Pond, Sandal, Yorks., 
Collected by Mr. J. Hebden. 
