GENUS LITHOTRYA. 335 



the rostrum sometimes appears like a few beads of a neck- 

 lace strung together. 



The Latera are remarkable from being placed over the 

 carinal half of the terga, in an oblique position, parallel 

 to the lower carina! margin of the terga. A section, 

 parallel to the growth layers, varies in the different 

 species from elliptic to broadly oval, and in Z. Nicobarica 

 it is triangular. Only a few layers of growth are ever 

 preserved. In Z. tru/icata, where the latera are repre- 

 sented by mere stiles, (like strings of beads), and are even 

 less in width than the rostrum, they are imperfectly cal- 

 cified. 



Microscopical Structure of the Valves. — The shelly 

 layers are white, and generally separate easily, so that in 

 Z. dorsalis it is rare to find a specimen with the upper part 

 of the valves perfect. The valves are so translucent, that 

 in the thin margins, even the tubuli could be sometimes 

 distinguished. The valves are coated by strong yellow 

 membrane, which, after the shelly matter in Z. dorsalis 

 had been dissolved in acid, separated into broad slips, 

 answering to each zone of growth. On the lower margin 

 of each slip, there is a row of closely approximate spines, 

 generally slightly hooked, pointed, ^th of an inch in 

 length, and i^th of an inch in diameter ; they arise out of 

 a little fold; all are furnished with tubuli of the same 

 diameter with themselves, running through the whole 

 thickness of the shelly layers, and attached, apparently, by 

 their apices, to the underlying corium. As the spines are 

 very numerous, so are the parallel rows of tubuli. After the 

 shelly layers had been dissolved, there was left in Z. dor- 

 salis (well seen in the latera), an extraordinary, conferva- 

 like mass of branching, jointed, excessively thin tubes, 

 sometimes slightly enlarged at the articulations, and ap- 

 pearing to contain brown granular matter : other portions 

 of the valves, instead of this appearance, exhibited mem- 

 branes or films with similar, branching, articulated tubes 

 or vessels attached to them : I have not seen this appear- 

 ance in any other cirripede. The yellow exterior en- 



