100 NOTES ON SOME ABNORMAL FORMS OF AULACOMSCUS. 



hyaline or minutely punctate areas separated by the inner ends of 

 irregularly bent flexuous strands of striae, sometimes continuous 

 with the space between the rows in the primary rays. An excentric 

 space of normal form and size is sometimes found at the inner ends 

 of the primary rays in A. excavatus from the Sysran deposit. 



5. Markings. — Variations are occasionally met with in the degree 

 of distinctness of the individual markings arising from their greater 

 or less elevation above the general surface. The examination of a 

 series of individuals of A.jormosus and A. margaritaceus, embracing 

 mature and immature valves, well illustrate these variations. The 

 arrangement of the markings sometimes shows striking irregularities. 

 In some abnormal Vera Cruz specimens of A. Kittoni the rows 

 between the approximated primary rays are almost parallel instead 

 of radial, in others a greater irregularity in size and arrangement is 

 found about the centre than towards the periphery. Occasionally, 

 too, considerable areas are entirely devoid of them, e.g., A. 7teyl<?ctu$, 

 and the interspaces, which afford a character of some importance, in 

 the discrimination of species varying in size and form, but, generally 

 speaking, are most inconstant in the vicinity of the central space, 

 e.g., A. wan/aritaceus, A. mammo.sus, A. intumescens mihi. Some- 

 times the markings are entirely absent from a narrow clear band, 

 of which the inner margin is irregular, the outer regular ; and this 

 is sometimes associated with changes in the position of the adjacent 

 markings, as in A. Comberi, or is unconnected with any further 

 modifications, as in A. marfjdritaceus and A. affmis. 



In the abnormal Chinese valve of A. ajjhris, already referred to 

 (p. 99), the markings become of normal form only on the outer 

 portion of the disc. Here they form short straight or regularly 

 curved rows, and are irregular towards the inner ends of the rows. 

 The outer edge of the elevated area is formed by a sharp dark 

 irregularly angular or lobate line, and the border is at some points 

 separated from the outer ends of the rows of markings by unequal 

 punctate areas, the largest bicrescentic at opposite ends of oile 

 diameter, the smallest lunate at opposite ends of the diameter at 

 right angles to the former. 



6. The Primary Eays.— The primary rays are an important cha- 

 racter in the present genus, yet their number is extremely variable, 

 —-from entire absence in abnormal forms of A. Kittoni to forty-five 

 in A. orientalis, — and they are far from constant, even in the same 

 species. Normally they proceed from the centre towards the cir- 

 cumference in straight lines. In A. Kittoni the rays are four or 

 eight on the surface of each valve ; they are formed of two 

 contiguous and conspicuous rows of angular markings, and terminate 

 in a considerable elevation provided with a siliceous cap that pro- 

 duces a well-defined dark crescent when the valve is next the 

 observer. Among the Vera Cruz specimens of this common species 

 are some with six rays, two beiii" approximated and curved in 



directions 



the 



space ; the markings in one row are sometimes larger than those 

 in the other (PI. 281, fig. 6). A second valve (PI. 281, fig. 5) with 



