82 



ART. 2.— H. MATSUMOTO : 



Fig. 24. Aströbon arcto.t. 

 b. From below. 



X li. «. From Mbove. 



The radial ribs are bar-like, 

 narrow, widest at the outer 

 end, suddenly narrowed for 

 a very short distance, then 

 uniformly tapered inwards, 

 nearly reaching to the cen- 

 tre. Interbrachial ventral 

 surfaces covered by a thick, 

 apparently smooth skin, 

 which contains very fine 

 granules. Adradial border 

 of the genital slits closely 

 spinulated. The madreporic 

 shield, situated at the inner 

 angle of the interbrachial 

 ventral surface, is more or 

 less semilunar, with a semicircular inner side and a distinctly 

 notched outer side, the lateral angles being rounded. The spaces 

 inside the interbrachial ventral surfaces are apparently smooth, but 

 in reahty closely covered with very fine granules of microscopic 

 size, the granules being rather coarse and distinct at the oral 

 angles. Teeth and papillae very numerous. The oral and dental 

 papillae are rather small, spiniform, not very acute, while the teeth 

 are much larger and longer than the papillae, and are distinctly 

 spatulate and flattened at the tip, the upper teeth being larger 

 and more pronouncedly spatulate than the lower. 



The two main stems of each arm are not equally developed, 

 but one is longer, stouter, and richer in bifurcations than the 

 »ther, as shown in the preceding table of dimensions. Dorsal 

 and lateral sides of arms covered by a thick skin, which is very 



