236 BRITISH AVONIAN CONODONT FAUNAS 
lateral denticles have been described by Branson & Mehl (1933), E. R. Branson 
(1934) and Scott (1961), from the Grassy Creek, Bushberg/Hannibal, and Louisiana 
Limestone Formations respectively. 
German workers recognized a progressive addition of denticles, firstly on the inner 
lateral face, and secondly on the outer lateral face. Forms with denticles confined 
to the inner supra-cavity position were referred to S. aculeatus (Branson & Mehl), 
whereas forms with a development of lateral denticles along the whole length of the 
unit, except for the high anterior blade, were referred to subspecies of S. costatus 
(E. R. Branson). Scott (1962 : 1224) separated as a distinct species (S. anteposi- 
cornis), forms with a single node immediately anterior to the oral surface of the 
cavity. 
From studies of the present British conodont faunas it appears that within the 
period Upper Devonian (to V) to Lower Carboniferous (Cu II «), there has been 
multiple development of laterally nodose spathognathodids. It appears that the 
Grassy Creek (to V) form S. aculeatus (Branson & Mehl) was the first species exhibit- 
ing this trend. No other to V forms with further development of lateral denticles 
on both lateral faces are known. Forms described from Germany and identified as 
S. aculeatus by Ziegler (1961) and other authors may be distinct from S. aculeatus 
and are probably a new species. 
The further developments of this species referred by Ziegler to S. costatus costatus, 
S. costatus spinulicostatus and S. costatus ultimus are definitely not representative of 
the species S. costatus (E. R. Branson) and S. spinulicostatus (E. R. Branson), and are 
named herein as new species, S. bischoffi sp. nov. and S. zeglert sp. nov. 
S. costatus costatus (E. R. Branson) (= S. costatus of Branson) and S. costatus 
sulciferus (Branson & Mehl) (= S. spinulicostatus of E. R. Branson) appear to be 
restricted to an upper Cu I-Lower Cu II « range, and can be seen to develop into 
forms referable to the genus Pseudopolygnathus. The fact that species of this genus 
are found at lower horizons (e.g. the Devonian-Carboniferous boundary) indicates 
that the genus Pseudopolygnathus is polyphyletic, the ancestral forms in both cases 
probably being nodose spathognathodids, as suggested by Voges (1959). 
Forms with from 2 to 4 lateral denticles, occurring with S. costatus (sensu E. R. 
Branson) and S. spinulicostatus (sensu E. R. Branson), appear referable to S. 
tridentatus (E. R. Branson), as distinct from S. tvidentatus (sensu Sannemann, 
Bischoff & Ziegler and Freyer), the latter forms being better referred to S. aculeatus 
(sensu Ziegler). 
In addition to these forms, there occurs at the base of the Lower Carboniferous in 
our sections, beneath and with the first occurrence of the genus Siphonodella, a new 
species of laterally denticulate Spathognathodus, with a plume-like anterior blade. 
Ziegler (1960A) described as S. tridentatus a form which appears referable to our new 
species S. plumulus plumulus. This species also exhibits evolutionary addition of 
denticles on the outer lateral cup but, rather than evolving to give species of the 
genus Pseudopolygnathus, it alters by lateral shift of the anterior blade into a homoeo- 
morphic form of the genus Cavusgnathus, named herein as a new genus Clydagnathus 
(p. 85). 
