56 MR. T. H. STEWART ON THE [Feb. 12, 



the teeth together, and in opposition to the next group of five (not 

 pairs), which pass from one radius to another internally and parallel 

 with the epiphyses of the alveoli, and assume, when connected with 

 the five radii, a' pentagonal form on the upper part of the conical oral 

 skeleton : these are the interradiales (PI. X. A. fig. 1 d), and act so 

 as to separate the points of the five teeth. But there are yet five more 

 muscles, of great importance to the object of this paper : these pass 

 from one alveolar pair to another, being attached to their serrated sur- 

 faces as short, coarse individual fibres : they are the interalveolar 

 muscles (PI. X. A. fig. 2 a), and their action is to move the points of 

 the teeth on each other in cutting the food. When a muscle is fixed 

 into a bone or hard substance, there is generally, I believe always, a 

 mark left of that attachment ; and this is the cause of these alveoli 

 being serrated as described. They are said by anatomists to be grind- 

 ing-surfaces ; but this cannot be, when the food cannot get near them, 

 and they are covered by muscles. When we examine this oral appa- 

 ratus in a fresh state, we find that the oesophagus is most firmly fixed 

 around the apices of the alveoli internally, where they are themselves 

 attached to the peristomal membrane ; and when the teeth protrude, 

 the oesophagus then takes its course through the centre of the conical 

 oral apparatus, being borne up by the five pairs of ligaments, each of 

 which passes as a broad band from the commencement of the oeso- 

 phagus, and is attached to the bifurcated inner ends of the falces. 

 Now, if the alveoli on their serrated surface or border were used to 

 grind the food, how is the food to get to them 1 It must pass through 

 the oesophagus, and thence to the rest of the alimentary canal ; and 

 for it to get near the alveoli, the alveoli must grind the oesophagus 

 itself. We may as well call the muscular impression on the shell of 

 an oyster or any other bivalve, or that caused by the impression of 

 muscles on our own bones, grinding-surfaces, as say that the alveoli 

 of the Echinus are grinding-organs. 



In the different genera that I have had the opportunity of exa- 

 mining, I find that there are certain peculiarities which might almost 

 form generic characters. 



Cidaris differs from Echinus in the form of the tooth : in trans- 

 verse section the tooth of Cidaris is semicircular or boat-shajjed ; 

 whilst the tooth of Echinus is somewhat T-shaped, having a ridge 

 running down the inner surface ; the alveoli in Cidaris are more 

 obscurely serrated on their opposed surfaces and oesophageal border 

 than in any other genus ; and the epiphyses of the alveoli do not 

 arch over and meet in Cidaris, as they do in Echinus ; and in 

 Cidaris the falces and radii are also shorter than in Echi7ius. 



In Biadema the shape of the tooth is like that of Cidaris ; the 

 alveoli are serrated on the opposed surfaces as well as on the oeso- 

 phageal border, and the serrations of the border are very long and 

 fine ; at the basal end of the symphysis on each individual alveolus 

 is a long hamular process, enlarged slightly into a flat free end, and 

 the basal border of the alveolus is long, which also necessitates the 

 epiphyses being lengthened. These epiphyses do not arch over, but 

 are like those of Cidaris ; the falces and radii are more like those of 

 Echinus. In the genus Acrocladia and Echinometra, at the end of 



