PATELLA. 255 



antero-dorsal position behind the prototroch, which has 

 now grown out and become the " Yelum," the differ- 

 entiated locomotor organ of the larva. The larva is 

 bound to its cap-shaped shell at first by a strand of tissue 

 inserted into the apex of the cap. Later on, Boutan has 

 observed in Acmcea the development of a pair of antero- 

 lateral muscular strands inserted near the shell margin 

 and functionally replacing the apical strand. This pair 

 of muscles spreads backwards, and ultimately form the 

 horse -shoe shell muscle of the adult. 



The shell now grows chiefly by additions to its posterior 

 border so that its mouth is much widened and the apex 

 comes to lie relatively far forwards. The apical portion, 

 which may show the beginnings of development of a 

 symmetrical spire, breaks off and the hole is closed by a 

 secretion of nacreous material. The next stage known 

 has the foot broadened into the adhesive sole of the adult, 

 the shell muscle horse-shoe shaped, and the shell conical. 

 This stage possesses a glandular streak on each side of the 

 foot, homologous with that found in the adult of NaceJJa 

 and its allies ; it disappears later. 



In further growth of the shell, the anterior margin 

 seems to share more largely, so the apex subsequently 

 undergoes relative motion towards the back. This is 

 more marked in limpets which live in positions much 

 exposed to the action of the waves, such limpet-shells 

 being lower cones than those in which the growth is more 

 equal. 



Conclusions. 



Of the features which have now been described in Patella, 

 the following characterise the Cyclobranchs, the specialised 

 group of the Docoglossa to which our type belongs: — 



(1) The highly differentiated crop placed so as to form a 



