86 AKT. 4. — N. YATSU : 



in breadth. The margin of the anterior half of the shell (PI. 

 VI., Fig. 86, and PI. VIII., Fig. 131) is dentated or sinuated, 

 owing to the uneven growth of the inner layer of the shell. 

 Morse found in the transparent plate of Glottidia pyramidata, 

 a young shell with a margin finely notched ('73, p. 260). 



The periostracum comes to view as a more distinct layer 

 than that in the preceding stage, measuring 1.6 p in thickness 

 (PL VIII., Fig. 130, pr. ort.). 



As the part of the peduncle lying between the valves is very 

 much flattened, there is no perceptible difference in the space 

 separating the valves at the hinge line, whether the peduncle is 

 protruded or not (PI. VIIL, Fig. 123). In either case the 

 hinge may be serviceable ; but in reality it has becomes of less 

 use, because the shells no longer gape at the anterior end as 

 widely as in the free-swimming larvae, and the arm-apparatus is 

 likewise not protruded out of the mantle cavity. 



Mantle and Setae. — In the mantle the amount of brown pig- 

 ment which was present in but a small quantity in the preceding 

 stages has increased in a zone slightly proximal to the margin (PL 

 VI., Fig. 86, and PL VIIL, Fig. 131). The large gland cells have 

 also increased in number, forming a distinct layer of a tolerable 

 breadth all around the mantle margin a little inside the pigment 

 zone just described, and constituting the gland ridge (Drüsenwall), 

 which becomes more prominent in the adult. This zone is best 

 seen in stained ^^-preparations (PL VI., Fig. 87, gl. ell.). The 

 mantle muscles become more distinct, running radially through 

 the marginal lacuna and in life they present a silky appearance. 

 The fibres branch at their entrance into the thickened border of 

 the mantle and terminate as fine fibres (PL VIIL, Fig. 131, r. m.). 



