288 GEOLOGICAL BIOLOGY. 



and Dayia outward toward the lateral slopes of the pedicle 

 valve (the position is intermediate between A and ) in 

 Catazyga toward the median plane just below the surface of 

 the brachial valve (7); in Glassia toward the centre, and the 

 apices nearly meet at the centre of the internal cavity (Z7) ; in 

 Cyclospira they are coiled nearly parallel to the vertical axial 

 plane, and the apices are slightly introverted. 



Although in lines of species (which in their combination 

 of characters show them to have close affinity and hence are 

 grouped in generic groups) the direction of the axis of the 

 spiral cone is pretty constant, we see that whatever impor- 

 tance may be attached to the different position of the spirals 

 in relation to the other parts of the body, the differentiation 

 of these features was quickly attained. 



FIG. 81 Diagrams of the various forms of the jugum in the Helicopegmata. a = Atrypina ; 

 t> = Spirifer ; c - Hindella ; d - Hyattella ; e = Retzia ; / = Whitfieldia ; g = Meristina ; 

 h Athyris ; * = Kayseria ; j = Meristella. 



The Form of the Loop. 5. The character presenting the 

 greatest degree of divergence in the structure of the brachid- 

 ium is the form of the loop or jugum. In the paper above 

 referred to, Mr. Schuchert has suggested that the nature and 

 complexity of the loop which joins the spirals are the more 

 important characters for subfamily differentiation. 



In Spirifer proper (Fig. 81, b) the loop is a simple band, 

 about the size of the primary lamellae, joining the two lamellae 

 together; in some cases in adults this was partly absorbed, 

 leaving only two calcareous processes facing each other on the 



