MONOGRAPH OF THE FAMILY 



FISSURELLIIXEi. 



Genus FISSURELLA. 



Char. Gen. Molluscum gasteropodum ; testa externa, patellee- 

 formi, dorsum tegente, netate ad apicem perforata, cicatrice nius- 

 culari, submarginali, crescentiformi, antice interrupts. 



Iu the very earliest stage of growth, the shell in all Fissurellldm 

 is subspiral with the margin entire. Then a passage is made for 

 the anal tube by a notch or sinus in the margin. In Scutus, or 

 Partnop/iorus, the sinus continues slightly marked to the margin 

 of the adult shell. In Fmarginula it is continued in the form of 

 a distinct marginal notch. In Cemoria, the margin closes round 

 the notch, leaving a hole near the apes, which is left entire ; in 

 Simula, the hole is left half-way towards the margin ; and in 

 Fissurella and the four following genera the spiral nucleus alto- 

 gether disappears, leaving a hole at the apex in its place more 

 or less enlarged by absorption. I adopt the proposed separation 

 from Fissurella, — 1st. Of Fissurellidea , because the shell is 

 covered by the animal ; 2nd. Of Cli/jndella, because it is peculiar 

 in form and only covers part of the animal ; 3rd. Of Macrochisma 

 for the last two reasons, besides the peculiar form and position of 

 the foramen ; and 4th. Of Fwplllma, on account of the insertional 

 ridge of the margin. I cannot agree to separate the larger num- 

 ber of remaining species under the generic name of Lueapina as 

 proposed, because the crenulatcd margin, the forward inclination 

 of the apex, the truncated internal callus and slight differences in 

 the fringes of the animal's mantle, are so much questions of degree 

 that it is practically impossible to divide the species so as to pre- 

 serve cousistencv in all the characters. 



