218 GENUS SCALPELLUM. 



the others in being at first developed under the form of 

 the so-called primordial valves : the other valves com- 

 mence by a small indistinct brown spot, very different 

 from the hexagonal tissue of the primordial valves : I 

 saw this very clearly in young specimens of S. vulgar e. 

 At first, the scuta, terga and carina, grow exclusively 

 downwards (and permanently so in most fossil species), 

 and therefore the growth of the scuta and carina is in an 

 absolutely opposite direction to what it is in Lepas, 

 Psecilasma and Dichelaspis. After a short period the 

 scuta are added to at their upper ends ; the portion thus 

 added, stands at a rather lower level, and projects in a 

 rather different direction from the first-formed part of 

 the valve, giving to it, in some respects, the appearance 

 of having been broken and mended. This structure is 

 common to S. vulgare, S. rostratum and S. Peronii. The 

 upper Latera (except in S. villosum) grow in the same 

 manner, namely, at first exclusively downwards, and then 

 both upwards and downwards. The rostral and carinal 

 latera (with the same exception of S. villosum) have their 

 um bones seated laterally, at opposite ends of the capitu- 

 lum, — the umbones of the rostral latera being close to 

 the rostrum, and those of the carinal pair close to the 

 carina, and consequently their chief growth is directed 

 towards each other. The carina in all the species, except 

 S. villosum, is either bowed or angularly bent ; in the 

 latter case the lower half is parallel to the peduncle, and 

 the upper half, extending far up between the terga, is 

 parallel to their longer axes. In some of the species the 

 carina is added to almost equally at both ends; in 

 8. ornatum it grows but little at the upper end, and to a 

 varying degree in different individuals according to their 

 age ; in 8. rutilum the umbo is at the apex, and there is 

 consequently no upward growth ; lastly, in jS. villosum 

 the carina widening much from the apex to the basal 

 margin, grows exclusively downwards, and a portion of 

 the apex projects freely, — characters all common to the 

 carina in the genus Pollicipes. The upper latera occur 



