PHOLADIDEA. 125 



figure of the adult. As yet unprovided with the calyx, its 

 posterior end, instead of being truncated, is rounded and 

 somewhat attenuated, whilst the dome-like structure of its 

 anterior extremity is replaced by a gaping expanse, above 

 which the side is obtusely angulated. 



This non-formation of parts and deferred enclosure of 

 the gape with shelly matter, until the last stage of growth, 

 is not confined to P. pap^/racea, but prevails likewise in 

 many Plioladidiz of the American seas. 



Dr. Turton's specimen of his Pkolas Imnellata is not 

 merely the young of this species, but seemingly a kind of 

 monstrosity, or at least a specimen of an unusual growth ; 

 for instead of the animal attempting as usual to enclose the 

 front ventral gape with testaceous matter, upon the com- 

 pletion of the immature stage of growth, it has contented 

 itself with solidifying the entire shell, and reflecting the 

 edge of its anterior ventral margin. This form is extremely 

 rare, but differs in no other respects from the ordinary 

 aspect of the young papyracea. There is a dwarf variety 

 figured in our plates which, instead of being littoral, is 

 taken by the dredge in detached lumps of rock five or six 

 miles from the shore. This is supposed by many collectors, 

 who regard lamellata as a distinct species, to be the true 

 young of papyracea, and not an adult form depauperated 

 by the unfavourable influence of an unnatural locality. 



Mrs. Griffith has kindly furnished us with coloured 

 drawings of the animals of the two forms taken from life, 

 and Dr. Battersby with good specimens of P. papyracea in 

 spirits. Mrs. Griffith regards both shells and animals of 

 each as indicating distinct species ; but the apparent dif- 

 ferences presented in the figures of the latter seem to 

 depend upon different states of the creatures at the moment 

 when pourtrayed, for the characters noticed as distinctive of 



