24 STRUCTURE OF THE CYSTIDE^. 



Niagara limestone last year, I procured at Thorold, near the Welland 

 Canal, a specimen which had been split in two from the apex 

 downward. I found only a portion of one of the halves. The 

 cavity of the body and proboscis had been filled with some soft 

 materia], which, upon exposure to the weather, had totally disap- 

 peared, thus exposing the structure of the inside of the cup as 

 distinctly as could be desired. 



Fig. 3. Fragmeat of Eucalyptocrmwi decorwi (Phillips), shewing six 

 of the ambulacral orifices. 



There are two orifices at the base of each arm, and consequently 

 twenty in all, as this species has ten arms. The specimen only 

 retains six. They are of an oval shape, about one line in length 

 and one-third of a line in greatest width. Each pair is separated by 

 a small elongated ridge-like plate or process, which cannot be seen 

 on the outside of the cup. All the inter-radials which are situated 

 in the same level with the apercures have at each of their upper 

 angles a sharp process, which projects inwards about half-a-line. 

 (In figure 3, if the dotted line from the letter a were continued, it 

 would cross the. centres of the apertures and inter-radials here referred 

 to.) The processes are above the line, and cannot be shewn by 

 wood-engraving. The small plates which separate the two pores 

 of each pair of the orifices have also each a similar process, between 

 which and the process of the contiguous inter-radial there is a very 

 narrow passage from the orifice upwards ; and it is possible that the 

 ambulacral canals took an ascending course by this route after 

 gaining the interior. Upon such points however of the internal 

 structure of the Crinoids, all that can be offered perhaps for an age 

 yet to come must be merely conjectural. The main fact proved by 

 this specimen is the existence of the ambulacral orifices at the base 



