Permian Producttnae and Strophalosiinae of W.A. 
39 
shell was attached at some period, but it is not known for what length of its 
life the shell maintained this position. If the shell was fixed only by cemen- 
tation the period of attachment was probably short as no specimens are 
known with the shell in this position nor are they usually broken across the 
umbonal region as one might expect. If, however, a process of the mantle 
aided cementation during life the shell would fall after the death of its in- 
habitant. There are no indications to show that such a process or processes 
existed, and one must conclude, in the absence of evidence to the contrary, 
that the Strophalosias were attached only during the earlier stages of growth. 
Remarks. — The large variation within the genus StrophaJosia is un- 
doubtedly due to the rate of adaptation in the different species to the seden- 
tary mode of life. 
In StropJialosia lamellosa var. linmbletonensis the shell shows a clear re- 
semblance to Productus in the arrangement of the musculature and brachial 
impressions. The adductor muscle impressions are central and the brachial 
impressions have a distinctly Productid aspect (see fig. 6, p. 38). 
If a form such as this assumed a sedentary mode of life with the point 
of attachment beneath the umbo, it is obvious that the structure is mechanic- 
ally unsound, as the muscles in -working, will be pulling against the point of 
attachment. To overcome this instability the muscles would tend to move 
backwards in the shell until they came to rest over the scar of attachment. 
This posteriorly directed movement of the muscles would push all those struc- 
tures posterior to them backwards, and these would thus tend to atrophy or 
be replaced by the originally more anterior elements. The backward move- 
ment of the muscles necessarily causes an alteration in the position of the 
brachial impressions, as already seen (see p. 38). 
In StropJialosia species with anteriorly placed muscle impressions, two 
oblique ridges arise, one on each side of the median septum, beneath the umbo 
and continue antero-laterally at an angle of about 45° to the hinge-line. As 
the muscles move posteriorly these ridges become displaced and gradually 
become the horizontal cardinal ridges as seen in Str. Jcimberleyensis mihi. 
They also impinge on the hinge-line so that the base of the cardinal process 
has different appearances according to the stage of movement of the muscles. 
Licharew (193*, p. 12 * ) states that the extreme condition of this move- 
ment is that shown by the Russian specimens in which the ridges have been 
pushed beyond the hinge-line, taking with them the sockets. The species to 
which Licharew refers, Str, fragilis and Str. horrescens, have dentritic ir.uscle 
impressions and must be placed as members of the genus Aulosteges. More- 
over, examination of the cardinal process of Str. horrescens shows that it has 
the typical trilobate structures and is not bifid, as might be expected if its 
mode of formation was as stated by Licharew. The backward movement of 
the ridges does change the form of the base of the cardinal process, but I 
have seen no specimen in which they replace it. The poor condition of the 
specimens of Str. clarkei figured by Etheridge (1880) led Licliarew to think 
that this series of changes took place within that species. Str. clarkei does 
represent an advanced stage, in that the muscle impressions are high and 
the ridges have come to assume a iiosition almost parallel to the hinge-line 
but the cardinal process has not been affected and the dental sockets are” deep. 
Comparison with other genera. — Strophalosia may be distinguished from 
both Aidosteges and Productus sensu lato by many features. In general, 
