30 ART. 2. — H. MATSUMOTO : 



vertebra. In very young specimens, the genital plates are very 

 high in position, lying nearly dorso -laterally to the first two ver- 

 tebra, and the lower muscular fossa3 of the vertebrsG are not 

 notably smaller than the upper, so that the genital plates and 

 vertebras rather remind one of those of the Ophiohyrsince. 



A most important internal character of the Trîcliasterînœ is 

 that the genital bursas communicate with the perihasmal canal. 

 In Astroceras, Trichaster and Euryale, I was able to observe this 

 communication ; and I behove that this character is a decisive 

 proof for the close relationship of these genera, as stated by Lyman 

 and Kœhler. In Astroceras pergamena Lyman, the genital bursas 

 communicate merely with the perihaemal canal, while in Trichaster 

 elegans Ludwig, they communicate also with each other in radial 

 pairs, the communications taking place above the vertebrae just 

 inside the disk margin ; in both the genera, the peritoneal cavity, 

 as well as the generative glands, is confined to the arm bases. 

 The skin of the arm consists of two layers, which are easily 

 separated by boiling in potash. The inner layei*, which is thicker 

 than the outer, contains the rudiments of the dorsal arm plates, 

 which are in smaller specimens very insignificant, being represented 

 by double rows of very feeble calcareous scales, continued above 

 fi'om the lateral arm plates, as stated by Lyman in Euryale aspera 

 Lamaeck. But in larger specimens, with the generative glands 

 extending into the arm bases, the dorsal arm plates become con- 

 spicuous in the proximal arm joints containing the generative 

 glands, so that the arm bases are strongly ribbed. At this stage, 

 each dorsal plate is represented by a pair of bar-like plates in 

 Astroceras, and by a pair of about triple rows of nodule-like 

 secondary plates in Trichaster. In Trichaster elegans, the genital 

 plates and scales are very stout, lying very closely side by side 



