38 
K. L. Pren'dergast. 
members of the genus shell formation was limited, probably by physical 
conditions, to certain times of the year, the periods of shell deposition being 
separated by intervals of (puescence during which no shell was deposited. 
That this response to physical conditions is a generic characteristic rather 
than due to abnormal physical conditions may be inferred from a study of 
forms living under the same physical conditions in which the lamellar struc- 
ture is not seen. 
Spines are often present; these may be recumbent or erect, many oi' few; 
they may be of two series or uniform in size, and are usually present on 
the pedicle valve, though but rarely found on the brachial valve. 
Internally, the structures of the brachial valve are characteristic of the 
genus, and the changes within the genus are largely reflected in the structure 
of this valve. A median septum is always present, varying in length from 
one-third to two-thirds of the length of the visceral portion of the valve. It 
separates the non-dendritic adductor muscle impressions which vary in posi- 
tion but shoAv no relation in their variation to the length of the median 
septum. The adductor impressions are usually divided into anterior and pos- 
terior adductors, but sometimes (e.g. Bramtonia) no such division is seen. 
The brachial impressions arise at the side of the adductor muscle scars 
and vary in position with the jmsition of the muscles. Where the muscles 
are medial or lower in the valve, that is, towards the anterior margin, the 
brachial impressions run almost parallel to the hinge-line and curve abruptly 
to the median septum, the condition seen in the Productus group. Where the 
adductors are situated higher in the valve the brachial impressions are found 
near the cardinal and lateral margins and approximately parallel to them. 
Figure 6. 
Internal Features of Brachial Valves — to illustrate the alteration in position 
of the Brachial Ini])ressions. (a) Productus latirostratus, (b) Strophalosia 
lamellosa var. humbletonensis, (c) Str. jukesi, (d) Str. kimberleyensis. (a), 
(b) and (c) after Davidson (Xli 2 )- 
The cardinal process, while it varies in minor details, retains throughout 
the genus, its typical trifid character. The process is always a compact one, 
the three parts being on the same horizontal level, in contrast with some 
Productids, e.g., Taeniothaeruf^ snbquadratus, where the central lobe is raised 
above the lateral lobes. The structures at the base of the process vary; 
this variation will be discussed later (p. 30). 
In the ]')edicle valve the adductor impressions are non-dendritic, central 
and varying in their position in the valve as do those of the brachial valve. 
The diductor impressions surround those of the adductor muscles and are 
longitudinally striate. 
In life the shells of this genus were probably orientated with the heavier 
pedicle valve resting on the substratum, the curvature of this valve and the 
upturned margins of the bracliial valve keeping the open edges of the shell 
above the level of disturbed water. The umbonal cicatrix shows that the 
