74 Mr. T. IT. Withers on 



Carina. — This valve is of the same length as the carinal- 

 latus and the apices of tliese two valves, together with that 

 of the tergum, form the upper extremity of the cnpitulum. 

 The valve is narrow, almost liuear, nearly flat, about the 

 width of the cariual plates of the peduncle, and there is a 

 corresponding valve on the opposing side of the capitulum. 

 Wyville Thomson has written in his description of S. mnc- 

 adami — " . . . this valve must be cither one of the valves of a 

 split carina — one of the parietes of a carina in which the 

 tectum is undeveloped ; or we must suppose the carina to 

 have been composed of two parietes and a separate tectum, 

 and the tectum to have been lost.^' In my opinion it is one 

 of the halves of a split carina in which parietes or iutra- 

 parietes had not been developed, and the valve is of the 

 same type of structure in <S. pulchellum and S. haworthi. 

 A ridge is invarial)ly formed along tlie median line in the 

 carinal valves of ordinary pedunculate cirripedes, and a 

 modification such as the splitting along this line would not 

 be unexpected. Such a secondary modification is seen in 

 the splitting of the dorsal plate in certain species of the 

 recent Molluscan genus Pholas. While such a modification 

 of the carina is quite unique among fossil and recent cirri- 

 pedes, a somewhat similar modification in the scutum is seen 

 in certain species of the recent genus Pcecilasma. The scutum 

 in that genus, as in the closely allied genus Lepas, lias the 

 umbo situated at the rostral angle, and the growth is entirely 

 upwards. In Lepas a ridge is formed on the scutum ex- 

 tending from the umbo to the upper extremity of the valve, 

 and running near and almost pai'allel to the occludent 

 margin. Essentially in the same position as the ridge in 

 Lepas, a suture is formed, whicli can be observed on both 

 surfaces of the valve in one speces of Poecilasma. The 

 development is carried a step further in other species of that 

 genus, for in tliose the scutum is definitely split into two 

 pieces. 



Peduncle. — This is al)out three times the length of the 

 capitulum, and in its upper part, just below the line of 

 junction, it is rather wider than the capitulum. It is com- 

 posed of ten rows of smooth calcareous scales, five on each 

 side, forming a most beautiful loiicated structure, sharply 

 pointed at its lower extremity. There are as many as twenty- 

 seven scales in a row in one of the specimens from Kansas, 

 l)ut the number naturally depends on the size and age of 

 the individuals (see immature example depicted on PI. IV. 

 fig. 1 A). The summit of the peduncle is usually somewhat 

 obliquely truncated, being lowest at the rostral end ; this is 



