214 D. M. S. Watson — A New Genus of Brachiopods. 



form a distiuct ledge or rabbet. Outside tbe flange the shell is 

 continued by a thin featureless adherent film to the irregular margin. 



The muscle -markings are restricted to the posterior end, in fact lie 

 largely between the dental areas. In full-grown individuals the 

 inner margin of the dental area of tbe left side (spectator's left when 

 the shell is placed with the hinge-line uppermost) is slightly excavated 

 for an abductor muscle, whose insertion is on a narrow area on tbe 

 side of the dental area, not reaching to the umbonal region, and in 

 two individuals clearly limited to a small raised area with a distinct 

 admedian margin. 



In most specimens there is no distinct area for the insertion of an 

 adductor on the left side, but in one or two where the shape of the 

 surface on which the shell grew has led to the existence of a small 

 hollow below the hinge-line and between the dental areas, this muscle 

 is represented by a small well-marked depression, separate from the 

 abductor insertion. 





Schematic figure of the posterior region of the ventral valve of Poikilosahos 

 petaloides. x 8. In this figure all the morphological facts to be seen in 

 the whole series of specimens are introduced. 



The adductor on the right side has a large and very well-marked 

 insertion which occupies a long narrow area passing forward from 

 the umbo nearly parallel to the inner edge of the dental area for 

 a considerable distance, sometimes a quarter of the shell length. 

 This muscle insertion is raised anteriorly on a special thickening of 

 the shell, the edges of which grow up round the muscle so that the 

 insertion lies at the bottom of a groove which may be of considerable 

 depth. The walls of this groove are so placed as to show that the 

 muscle passed obliquely across to the dorsal shell and was attached to 

 it either on the middle line or beyond it on the left side. There is 

 no evidence of the existence of an abductor muscle on the right side, 

 as there is no such excavation of the dental area as forms its insertion 

 on the left side, and the outer wall of the thickening for the insertion 



