DEVONIAN FOSSILS 101 
the margin of the shell, so that the costellae of the external surface 
are not shown as is so often the case with strophomenids. The 
papillae cover much of the median septum between the muscle 
scars, and also run up the ridges effecting the tripartite division 
of the muscle areas on each side of the septum. 
Description of Paratype. Dorsal valve more or less flat with 
holocrenulate hingeline ; there is no interarea apart from the teeth 
file. Small quadrifid sessile cardinal process, scarcely if at all 
extending beyond the hingeline. The two central prongs of the 
process are elongate, being about 1-5 mm. long and 0-5 mm. wide. 
They are but slightly splayed apart. On each side of these larger 
prongs, almost at the hingeline, are much smaller ones. Crural 
bases obselete. Adductor scars small, posteriorly situated, the 
rims forming a pair of inverted U-shapes. Stronger papillae 
occur on each side of these scars than occur on the rest of the 
inner surface of the valve. 
Comment. Other specimens on the same slab as the types indi- 
cate that the ornamentation of the external surface is costellate. 
The new species varies in proportions, but it is difficult to make 
satisfactory measurements owing to the crushing which the matrix 
has suffered. There are variations also in the length and width of 
the ventral median septum, although the specimens available hint 
that there may be two distinct varieties. More material is needed 
to determine this with certainty. The extension of the papillae to 
the edge of the inner surface of the ventral valve indicates that 
secondary deposition occurred over the whole of the interior of the 
valve, a condition which contrasts with that usually observed in 
strophomenids. 
The large muscle field with its strongly developed ridges in the 
ventral valve is a mark of an advanced form in a genetic sequence, 
and may be compared with similar structures in Hipparionyz. 
The early protoleptostrophids show no division of the ventral 
muscle into bundles (e.g., P. plateia from Tasmania—Gill, 1948). 
Then follow forms in which a tripartite division is present, as 
in the new species described above. Finally, there is the group 
covered by the genus Leptostrophia in which a further division 
has taken place, giving six muscle bundles. Since Prolepto- 
strophia is found in both Lower and Middle Devonian, while 
Leptostrophra, although a specialized form, existed only in the 
Lower Devonian, it is to be inferred that Leptostrophia was an 
offshoot from the main line of development. It appears to have 
ended in a cul-de-sac, while the less specialized Proleptostrophia 
gave rise to other forms. i 
