GENUS ELMINIUS. 345 



forming nearly a straight line with the margin on the carinal side. In 

 some young specimens, about the tenth of an inch in diameter, from 

 the West Indies and from New South Wales, the spur (the position of 

 which I found varied a little in some other specimens) was placed 

 nearly in the middle of the valve, and very nearly at right angles to the 

 basal margin ; it is possible that these may be a distinct species, but 

 without larger specimens to judge from, I think it more probable that 

 this difference in the tergum is due to variation and youth. 



Structure of Shell and Radii. — The parietal tubes are commonly 

 elongated in the ray of the circle : the septa are rather thick, and 

 strongly crenated at their basal edges. The inner lamina of the walls 

 is strongly ribbed longitudinally. The broad radii have their sutural 

 edges formed by ridges, with numerous and closely approximate denti- 

 culi : the interspaces between the main ridges are not soon filled up, 

 and at the bottom, each interspace usually terminates in a pore or 

 tube; so that the radii are not solid, as in most of the foregoing species, 

 but porose. The alee have their edges crenated. 



Basis, calcareous, of unusual thickness ; the inner, or upper surface, 

 is striated from the centre in rays, corresponding with the ribs on 

 the inner lamina of the walls. This striated or furrowed structure in 

 the basis, shows a tendency to its becoming tubular or porose, as may 

 be inferred from analogous cases in Balanus. 



AnimaVs body unknown. 



Affinities. — This species is rather more closely allied to the last than 

 to any other. There is a close analogy in the peculiar manner in which 

 the scuta and terga are articulated together in the two species : in 

 this species it is effected by the great development of the articular ridge 

 of the tergum, and in T. ccerulescens by that of the scutum. The inter- 

 nally striated calcareous basis, and the internal tubular interspaces 

 between the denticulated ridges of the radii, are peculiar characters. 

 The white colour, the narrowly and closely ribbed parietes, and the 

 broad radii, give this species an aspect, by which it can be easily 

 recognised. 



4. Genus — Elminius, 



Elminius. Leach. Zoological Journal, vol. 2, July, 1825. 



Compartments four : parietes not porose. Basis mem- 

 branous. 



Distribution, Southern temperate seas. 



General Appearance. — Shell conical, with a strong ten- 

 dency in most of the species to become cylindrical : orifice 



