8 KULLETIN (iO, UNITED 8TATES NATIONAL MUSEUM. 



cc. Inframedian latus small or narrow, triangular, hourirlass-ishaped, or 

 irregular. 

 (I. Rostral latus low, usually twice as wide as high. 



IV. Section HoloKntlinlhnn, ]>. 25. 

 eld. Rostral latus usually as high as wide, with short l)asal margin. 



V. Section — p. 47. 

 hh. Plates imperfectly calcified, at least the terguni V-shaped." 



VI. Section Neomalpelluvi, p. 09. 



Key.s to the species described in this report, and incidental 1\- to all 

 known North American forms, are given under each sectional head 

 below. 



The subgenera Oalantica and Sinilium should, in my opinion, be 

 elevated to the rank of genera; but as onl}'^ two species of Oalantica and 

 none of Smilium are described in this paper, I have followed the cur- 

 rent generic arrangement. Both groups are more primitive than 

 Scalpellum^ and Calantica was probably ancestral to Scalpellum. and 

 Stniliuin. 



Sections IV and V are mutually more closely related than the 

 others, yet, as they seem to constitute natural phyla and are very 

 numerous in species, I have retained them separate. The rostrum 

 may l)e either present, vestigeal, or absent in the last four groups. 



I. Subgenus CALANTICA Gray. 



1825. Calantica Gray, Annals of Philosophy, n. ser., X, p. 101, for S. viUoftiDii 

 Leach. 



The capitidwn has two whorls oj^ plates, the upper consisting of scuta, 

 terga, and carina, the terga occujyyiiig all the space hetween scuta and 

 carina' lowd' whorl consisting of three pairs of latera, rostrum, and 

 sahcarina. ZJmhones of all the plates are apical. Cowiplemental males 

 with a distinct capitulum furnished with valves and a peduncle. Type, 

 S. villosum. 



The capitulum closely resembles that of Pollicipes, yet differs (1) b}^ 

 greater specialization of the armour, there being but a single basal 

 whorl of plates, and (2) by the presence of complemental males, as 

 in Scalpelhivi and Ihla, yet these males resemble miniatures of the 

 hermaphrodite form. Scalpellum differs from Calantica chiefly ])y 

 the elevation of the lateral plates, which become, as "upper latera," 

 members of the upper whorl of plates, and thereb}' modify the relative 

 positions and shapes of the terga and scuta. In the t3^pical forms of 

 Calantica the median lateral plate can not properly be called an "upper 



n Section VI, as defined in this key, would include forms of diverse ancestry, but 

 parallel in the one character of reduction of the calcified portions; and in actual 

 practice all such species would have to be given place in a key to the species of this 

 section, though they should be grouped with their real allies. In dealing: with 

 parallel phyla it is probably impossible to con.struct a key for tiie convenient deter- 

 mination of specimens without using artificial characters. 



