146 



Mr. E. Billings on the Structure of 



between two of tlie pores. On the other hand, a small groove 

 runs from each pore inward, and terminates at another socket 

 about halfway between the pore and the main median groove 

 of the ambulacrum. It would thus appear that, besides the 

 ordinary pinnules, there were two other rows of appendages 

 on each side of the median groove. 



The general conclusions at which I have arrived from the 

 above are — that all the principal plates that compose the shell 

 of Pentremites belong to the perisomatic system of Professor 

 Wyville Thomson, that it is doubtful whether or not the 

 lancet plates are homologous with the radial plates of the Cri- 

 noids, and that the ambulacra are more complicated in their 

 structure than is generally supposed. 



7. On the Structure of the Genus Nucleocrinus. 



The body of this remarkable genus is ovate, elliptical, or ob- 

 long, and enclosed in a shell of strong perisomatic plates, Avliich 

 are in general so closely anchylosed that the sutures between 



Fig. 3. Fig. 4. Fig. 5. 



'<//A^^^' 



Fig. G. 



^. A A^-^A A '-&/ 





Fig. 3. Apex of Niideocrimis J'crnnnlii\ Troost : r/, ambiilacral groove ; 

 p, pore tln-ough -which the groove enters into the interior ; s, one of the 

 ten spiracles; mv, oro-anal aperture. Fig. 4. Anterior side of a speci- 

 men : o, the anterior interradial. Fig. 5. Apex of a specimen which 

 has lost the integument that covered the centre. Fig. 6. Diagrams of 

 the plates of the test : a, ambulacral plate ; b, the basals ; c, plates of 

 the apex ; d, one of the interradials ; /, forked plate. 



