NOTES ON SOME ABNORMAL FORMS OF AULACODISCUS. t) ( J 



A valve of A. exeavatas Sch., from Sysran deposit, has the sur- 

 face sloping uniformly outwards from the process placed farthest 

 from the margin; the compartments between this and the two 

 remaining processes are normal, but the third is devoid of the 

 shallow basin-like depression characteristic of the species. 



3. Colour. — Differences of colour in mature valves depend on 

 the thickness of the valve, those having the superficial layer absent 

 from certain portions being lighter there than elsewhere, e.g., A. 

 rotxdus mihi and A. Comberi, on the size and proximity of the 

 puncta, on the spaces between the markings and on the central 

 space, e. g., A. orientalis Grev., and on the delicacy of the markings, 

 e.g., A. exiguus, A. crux Ehrb. In an abnormal specimen of A. 

 margaritaceus Ealfs, from Pisagua, the surface is much mottled in 

 appearance, the intensity of the bluish to dark greyish hue varying 

 in different places. In abnormal valves of A. Kittoni, as in normal 

 specimens of A. Rattrayii, darker irregular radial areas are very 

 evident. Hyaline valves of normally darker frustules are not 

 uncommon in A. Oregonus Harv. & Bail, and 0. orientalis. 



4. Central space. — In some species the central space is uniform 

 in outline and dimensions, e.g., A* Oregonus. A. orientalis, but in 

 others the form is variable, e.g., A. margaritaceus. In A. exiguus 

 it normally has narrow extensions outwards between the primary 

 rays, and wider lobate extensions have sometimes been observed in 

 A. formosus. The limiting markings may be regular or irregular in 

 the same species. In abnormal Vera Cruz specimens of A. Kittoni 

 two minute irregular subcentral spaces separated by a single 

 irregular row of markings sometimes occur, the one being at the 

 inner ends of the widest adjaceut primary rays, the other at those 

 of the remaining rays. Another specimen of A. Kittoni from the 

 same locality (PL 281, fig. 7) has two single rays, and two double 

 ones united in the same crescent and occupying one elevated area. 

 The single rays and one of the double rays meet in a subcentral 

 rosette, another of the double rays stops short of the rosette, and 

 the remaining rays originate in an elliptical excentric clear space. 



A further modification occurs in the specimen represented in 

 PI. 281, fig. 8, also from Vera Cruz, where there are two equal sub- 

 central rosettes in contact with each other and placed in line with 

 the two opposite straight primary rays. On the other hand, the two 

 rosettes may be inconspicuous, separated by a few irregular markings 

 without interspaces (PI. 281, fig. 4), or a distinct rosette and central 

 space may be absent and replaced by an irregular excentric area 

 crossed by irregular curved bands (PI. 281, fig. 2, 3). 



In Pisagua specimens of A, nmrqaritacem, and in others found 

 b J the * Gazelle' expedition, two subcentral spaces have been 

 observed. In the former these are markedly different from one 

 another, the one elongate and placed at the inner end of six of the 

 primary rays, the other smaller, quadrangular at the inner end of 

 tbe seventh ray. 



A specimen oiA. a {finis Grun., found by Macrae in Holothurians 

 fr om China and now in the collection of Kitton, has the central 

 s Pace replaced by seven large and a few intermediate smaller 



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