286 GEOLOGICAL BIOLOGY. 



showing the sudden reflection and twist in the origin of the 

 primary lamellae among the first species of the Eosilurian (viz., 

 Dayia, Hindella, Merista, etc.), while the Atrypidae and Spir- 

 iferidae, in which the lamellae are directly 

 continuous with the crurae, are fully ex- 

 pressed at the base of the Upper Silurian. 

 ^ e secon d particular in which difference 

 * s exm i te d is seen in (b) the direction away 

 --> from or else parallel to the plane of a me- 

 A "AT dian se P tum - I n one extreme (see in Zygo- 



' V\ spira, Figs. 73-76) the lamellae diverge at 



a right angle (or less) from the extremity of 



FIG. 79. Athyns, showing S & \ J 



ff twVcr e uri rac i idi si^- t ^ le crurae toward the lateral borders of the 

 twisted pit 2Khe p ri- shell, and curve outward and thence down- 



/unSg ward along this outer border to the front; 



hVthe and as they reflect in the course of the first 

 volution, turn inward toward the centre. 



follow the direction and T , , . . 1 i_ j-t, 



lie upon the upper part In this type the spirals have their apices 



of the primary lamellae. , . , 1 , A 



directed more or less inward. Atrypa pre- 

 sents these characters, and Schuchert has adopted the charac- 

 ters of its brachidium as a mark of the family Atrypidae (see 



p. 279). 



In Spirifer the lamellae proceed with almost no divergence 

 in two nearly parallel lines, from the extremities of the crurae 

 directly toward the front along the inner surface of the 

 brachial valve, and at the front curve directly toward the 

 pedicle valve, and in making the first volution of the spiral 

 return nearly to the starting-point at the end of the crurae. 

 The spiral thus formed has its apex directed outward toward 

 the lateral border of the valve, and it is in this type of 

 brachiopods that the great production of the lateral wing of 

 the shell takes place, and the apices of the spires penetrate 

 into the pointed extensions of the shells (see Figs. 77, 78). 

 These two extreme types, however, first appear near together 

 at the veiy base of the Upper Silurian. 



The Number of Volutions of the Spiral. 3. Another diverg- 

 ence is in the number of volutions of the spiral. The earliest 

 known Helicopegmata are generally of small size, and the 

 volutions are not numerous: it is not improbable that the 



