652 TRILOBITES OF THE UPPER SILURIAN ROCKS. 



to the end of the ribs. Lateral portions of ribs falciform, and descending abruptly, the points 

 being directed anteriorly. 



Caudal portion (pygidium 1 ) very distinct from the body, ribs 12, lateral lobes nearly equal 

 in width to the central lobe. Tail smooth and acuminated. 



The genus Homalonotus is distinguished from the Calymene by the much greater propor- 

 tional width of its central lobe, as well as by the apparent absence of trilobation and the 

 peculiar structure and form of the head and tail. It is also remarkable in the falciform termi- 

 nation of its lateral ribs or segments. 



The suture (linea facialis) which divides the head into 3 parts, and which, according to 

 Dalman and other authors, is a distinguishing feature of many Trilobites, is well developed 

 in the Homalonotus delphinocephalus , and is also seen in the Bumastus Barriensis (nob.). 

 In the latter, however, this line passes under the anterior part of the head as shown in PL 7 bis, 

 f. 3e. This suture is rarely seen in our English specimens owing to the nature of their 

 matrix. 



The whole of the surface of the Homalonotus was scabrous, but this character is best seen 

 in those portions which were apparently least exposed to friction while living, as at the ends 

 of the side lobes, particularly those of the " pygidium " ; for on the back or middle lobe the 

 surface is worn into slight indentations. (Fig. 1 c. PI. 7 bis, exhibits a portion of the surface of 

 the post abdominal side lobe; f. d. a portion of the central lobe, both magnified.) 



The smooth parts intermediate between each segment present a different aspect in different 

 specimens, being almost entirely covered by the ornamented portion of the ribs in some (see 

 the 12th and 13th body ribs f. 1 a and b.), and in others appearing, even in the centre of the 

 body, as considerable ridges nearly half the width of a rib. This sculpture, partially seen in 

 the anterior part of the body of f. 1 a and b., and more clearly developed in other specimens 

 of the same species found at Dudley and also in the II. Knightii and II. Ludensis, PL 7. 

 figs. 1, 2, 3 and 4., shows that the animal must have had the power of contracting and ex- 

 panding by moving the ribs over each other. Thus the lateral termination of these plates is 

 seen to be scabrous, while their central parts over which the ribs worked ? are smooth, as may 

 be seen in some sculptured crustaceans which have the power of coiling up their abdomen. 



As I can discover no difference between our Homalonotus and the American Trimerus del- 

 phinocephalus, (Green), except in size, (the latter being very diminutive) I have retained the 

 specific name of the American author, while I adhere to the generic name of Konig, which 

 was applied to bodies of this form before the publication of Dr. Green. 



Loc. Dudley Castle. (In the upper beds of the Wenlock Limestone.) I am indebted to 

 Mr. Blackwell for the loan of the fine fossil PL 7 bis, f. 1., and also to Mr. John Gray and 

 Mr. Morris of Dudley, for the use of instructive portions of the head, body and caudal portion, 

 which enabled me to describe some of the above-mentioned peculiarities. 

 Homalonotus Herschelii (n.s.), PL 7 bis, f. 2. 



Differs from H. Knightii, H. Ludensis and II. delphinocephalus, in the body being covered 

 by strong and prominent tubercles, and in the posterior sides of the head being terminated 

 laterally by a double-headed short process. In all the essential characters of the genus, how- 

 ever, namely, the indistinct semblance of trilobation, the ovate- acuminated head, form and 

 position of the eyes, number of ribs and their falciform terminations, together with the 



1 I have for the most part employed the terms used by Dalman to designate the different parts of the body. 



