﻿120 BRITISH FOSSIL CEPHALOPODA. 



slightly oblique, about 5° in a direction contrary to the ornaments, which often pass 

 across from one septum to the next. Their distance, as seen in the specimen figured 

 by Portlock, as 0. subflexuosum, is £ the diameter ; their convexity is |- of that line. 

 If a small septal surface is rightly associated with this species, the siphuncle is -f- the 

 long diameter from the side. The diameter is usually less than ^ an inch, but the 

 length is considerable, so that the whole shell is a small and delicate one. 



Relations. — As will be seen from the synonymy, these finely-marked Orthocerata 

 from the Lower Silurian rocks have been separated into several species, but their 

 limits have never been well defined. The smaller number of riblets has been made 

 use of as a character for 0. tenuicinctum, but there are graduations in this respect so 

 close, as to make it impossible to draw a satisfactory line. A more hopeful means 

 of separation is by the amount of the undulations of the ornaments, but I have not 

 found any in which they do not undulate at all. The riblets are far less numerous 

 than in 0. argus. A very similar shell is 0. expansum, when the details are con- 

 sidered, but it is altogether a robuster form, and the septa are more distant and the 

 rate of increase greater. From 0. undidocinctum, on the contrary, it differs by the 

 greater number of its riblets, and from 0. recticinctum by their undulation. 



Distribution. — The greater majority of examples are from the Bala Beds of 

 Desertcreat (12), but it occurs also at Kildare (3) and Coldwell (2). 



It is recorded by Baily from thirty-seven localities in Tipperary from the 

 Lower Silurian (see Explanation of Sheets 133, 135, 145). M'Coy records it from 

 Lower Silurian, Galway ; and Lapworth from the Middle Silurian, Gala. 



A minute specimen (PI. XIII. fig. 10), in the Woodwardian Museum, from the 

 Bala Limestone, Dent, about 1 inch long and 2 lines in diameter, shows a beautifully 

 cancellated surface. The transverse riblets are 15 per line, and the longitudinal 

 about the same : the latter look as if they were dying off ; and as many species 

 have longitudinal lines in youth which are subsequently lost, it is very probable 

 that the specimen may represent only the young of the present species, with which 

 it agrees in general character. It stands, however, in the Museum Catalogue as 

 "0. conularia." 



Orthoceras ttndulocinctum, Blake, PI. XIII. fig. 9. 



Type. — The shell is compressed, so that the axes are in the ratio of 3 to 2. The 

 rate of increase is about 1 in 6. The ornaments are transverse sharp riblets, which 

 are oblique in the contrary direction to the septa, and cut across two chambers 

 in crossing from side to side. These are about 7 per line. The septa are only 

 seen to undulate on the surface exposed, but this is probably due to their obliquity ; 

 they are distant -^ the diameter. The siphuncle is not seen. The length is 

 If inches, and the greatest diameter § inch. From the Upper Ludlow, Ledbury. 

 In the Museum of Practical Geology. 



