HOMONOTUS. 27 



well exposed in B. M. no. o9074, and the separate laminar nenral spines at the 

 base of the caudal fin are seen (PI. VII, fi^-. (>). 



In the pectoral arch the clavicle is much expanded above the base of the fin, 

 and this exposed lamina (PI. VII, figs. G, 7, cJ.) is ornamented with fine rugse 

 which pass into coarser pectinations at its hinder border. Only slight traces of 

 the pectoral fin have been observed, and it must have been small and delicate. The 

 pelvic fins are inserted directly beneath the pectoral pair, and distinctly comprise 

 seven robust rays, of which the foremost seems to be a stout, longitudinally ridged 

 spine (PI. VII, figs. 6, 7,2)1 v.). The length of these rays is indicated by remains 

 in the original of PI. VIII, fig. 7 ( jj/t'.). The median fins are only known by frag- 

 ments, and the total numljer of ra3*s is uncertain. The counterpart half of a 

 specimen in the Dixon Collection (B. M. no. 25910) appears to show that there are 

 12 dorsal fin-spines, all slender and impressed with at least one longitudinal groove. 

 The first 5 or (> spines gradually increase in length ; and the length of the others 

 equals about half the maximum depth of the trunk. The proportions of some of 

 these spines are shown in PI. VII, figs. G, 7, and PI. VIII, fig. 7. The spines of 

 the anal fin (PI. VII, figs. G, 7, a.) are rather stouter than those of the dorsal fin. 

 They are four in number, with separate supports (PI. VIII, fig. 7, a.), and 

 gradually increase in length to the fourth, which is not so long as the longest 

 dorsal spines. The articulated rays in the dorsal and anal fins are scarcely known, 

 but two specimens show that the extent of the anal fin equalled at least the length 

 of nine vertebrae. The caudal fin is also known only by fragments {c.'j., PI. VIII, 

 fig. 7, t'.), but it must have been rather large and forked. 



The scales (PI. VII, fig. 6 a) are large, thin, and deeply overlapping ; none 

 extend OA^er the head or fins. Their number seems to be about thirteen in a trans- 

 verse series on the flank of the abdominal region. Their hinder border is gently 

 rounded, and their exposed face is marked with radiating pectinations, which are 

 most conspicuous on the ventral scales of the abdominal region. As shoAvn by the 

 original of PI. VII, fig. 7, the ventral border is rounded, Avithout modified ridge- 

 scales. The course of the lateral line is marked by a rounded ridge on each of the 

 scales traversed, and this ridge is still prominent quite at the end of the caudal 

 pedicle (PI. VII, fig. 7). 



Horizons and Localities. — The type specimen seems to liaA-^e been obtained from 

 the zone either of lihynchonella Giivieri or of Terehratidina gracilis at Mailing, near 

 Lewes, Sussex. The small specimen in the British Musevmi, already mentioned, 

 was probably found in the zone of Holaster suhglohosus at Burham, Kent. The 

 species, however, occurs more commonly in one of the higher zones of the Chalk, pro- 

 bably in that of Micmdrr coranguinum : Bromley, Northfleet, and Grravesend, Kent ; 

 Micheldever, Hampshire. It has also been obtained by Dr. H. P. Blackmore from 

 the zone of Actiuocamax quadra.tus at East Harnham, and from the Uintacrinus- 

 band of the zone of Marsiijnfc^ near Salisbury. 



