D. M. S. Watson — A Nexv Genus of Bracliiopods. 217 



central channels running antero-posteriorly. Each of the lateral 

 canals is open to the outside by a slit running the entire length of the 

 dorsal pinna, formed by the gap between that pinna and the one 

 behind it. If we suppose that the ciliary currents are driven down 

 these lateral canals and by the action of the strip of lophophore along 

 the side of the median septum are urged towards the posterior end, 

 we have a feeding arrangement which, though not exactly paralleled 

 by anything we know, should have been as efficacious as we infer 

 from the immense size of Lyttonia it must have been. 



Since the time of Waagen it has been customary to refer Oldhamina 

 and Lxjttonia to a special sub-family Lyttoniinse of the family 

 Thecideidse. This view has been accepted by subsequent authors, 

 although only with great reserve by Noetling. The much less 

 specialized Poilcilosalcos affords better material for comparison with 

 Thexideas than the later Oldhamina. Poikilosaleos and the true 

 Thecideidse have in common only the attachment of the ventral valve 

 and the lobed lophophore. 



The attachment of the ventral valve is not a feature of great 

 systematic importance, as it has repeatedly arisen in Brachiopods. 

 The lobed lophophore if strictly comparable in the two groups would 

 be of much greater importance, but the early Lyttoniids, both 

 Poikilosaleos and Keyserlingina, show that the lobingof the lophophore 

 in that group has developed in consequence of a notching of the edge 

 of the dorsal valve, whilst the early true Thecideas, of which the first 

 large series are of Upper Liassic age, show no trace of any incision of 

 the margin of the dorsal valve, but suggest that the lobing of the 

 lophophore has arisen in them through the upgrowth in the dorsal 

 valve of true septa quite analogous with the median septum which 

 in the life-history of all Terebratellidse and Dallinse notches the 

 anterior margin of the primitively circular brachideal band. 



There is thus no evidence at all that the Lyttoniidse and the 

 Thecideidse are allied. The two groups differ markedly in the very 

 early and complete loss of an area and delthyrium in the former and 

 its retention in a typical form by the latter group, and in the great 

 reduction of the muscles together with a probable loss of power of 

 opening the shell in the earlier family contrasted with a highly 

 developed musculature and a power of opening the shells at right 

 angles in the Thecideidse. It is thus necessary to place the two 

 groups in independent families and to discuss their relationships 

 separately. 



The interesting Brachiopod Pterophloios emmericlii from the Kossen 

 Beds deserves to be considered in the light of this distinction 

 between the Thecideidse and Lyttoniidse. I know it only from 

 a single dorsal valve in the British Museum, and from Zugmayer's 

 figures. In shape it resembles a small Oldhamina, having strongly 

 involute valves and being attached only by a small portion of the 

 umbonal region. It has neither area nor deltidium. The "septa" 

 which occur in both valves split at their outer ends, and form 

 a continuous band following the margin of the shell, and much more 

 resemble the septa of the ventral valve of Oldhamina than of any true 

 Thecidia. It differs from every known Lyttoniid in, haviDg a solid 



