LITHOTRYA RHODIOPUS. 365 



out on both margins. The growing corium-covered sur- 

 face (fig. 4 b) is transversely elongated, with the occludent 

 margin rounded, and the carinal angle much produced, 

 but not forming a roughened knob. 



Carina (fig. 4 d), concave within, with a slight central 

 ridge in the upper free portion. The inner growing sur- 

 face is concave, almost pentagonal, with a just perceptibly 

 raised central rim in the upper part, and with two minute 

 prominences on each side, against which the produced 

 carinal angles of the terga abut. 



Mostrum t lost. 



Latera (fig. 4 c), growing surface (or a section parallel 

 to the growth-layers,) symmetrically oval, more than one 

 third as wide as the basal margin of the carina. Several 

 zones of growth preserved. 



Peduncle, lost, but a few scales accidentally adhering 

 to one of the valves, show that they are crenated in the 

 three or four upper whorls. No basal calcareous cup was 

 preserved, but by clearing out the base of one of the holes 

 in the coral, in which a specimen had been imbedded, I 

 found a little fiat disc about the size of a pin's head ; it 

 was composed of two or three layers, and was externally 

 coated by yellow membrane, including the usual spindle- 

 shaped bodies and tubuli. The cement-ducts were also 

 discovered after dissolution in acid. So that there could 

 be no doubt regarding the nature of the little disc. 



Mouth. — Labrum with a row of little blunt teeth. 



Palpi, blunt, rather expanded at their ends, with the 

 extreme margin much arched and furnished with two 

 rows of long spines ; there is a fringe of short spines on 

 the straight inner side. 



Mandibles. — There are nine pectinations between the 

 first and second main teeth, and only two between the 

 second and third teeth; the inferior angle is coarsely 

 pectinated, with one central spine twice as long as the 

 others. The distance between the tips of the first and 

 second main teeth, is greater than between the tip of the 

 second tooth and the inferior angle. 



