97 



NOTES ON SOME ABNOEMAL FOKMS OF 



AULACODISCUS Ehrb. 



By John Rattray, M.A., B. Sc, F.R.S.E. 



(Plate 281). 



w 



In the course of investigating tins genus for the Monograph 

 which I have just laid before the Royal Microscopical Society, 

 I have met with a number of abnormal forms, which it seems to 

 me desirable to record and describe. The genus is characterised by 

 the circular outline of its valves, by the surface — almost flat or with 

 a somewhat elevated band in the course of the radius ; by the free 

 circular or closely approximated angular markings, and by the 

 presence of distinct processes at the ends of the primary rays, 

 which proceed outwards from a hyaline or punctate central space 

 or from a central rosette. 



The abnormalities which I have observed affect the outline, the 

 undulations of the surface, the colour, the character of the central 

 space, the markings, the number and direction of the primary rays, 

 and the processes. 



1. Outline. — With the single exception of A. polygonum Grun., 

 which, as its name implies, is many-sided, all the species of this 

 genus have a circular outline. In valves of A. Kittoni Arnott, how- 

 ever, the margin sometimes becomes more or less straight, so as to 

 produce an obtusely angular form, suggesting the normal more 

 rectilinear-sided specimens of A. polygomis; and in a considerable 

 number of species I have met with valves that are more or less 

 regularly elliptical, e.g., A. Barbadensis Rails, A. angulatus Grev., 

 A. elegans Grove & Sturt, A. disperstis mihi, A. amamus Grev., A. 

 affinis Grun., A.forwosm Arnott, A. aucklandicus Grun., A. Petersil 

 Ehrb., A. Kittoni Arnott, A. Eattrat/ii Grove & Sturt, A. SoUittianus 

 Norman. A more local interference with the circular outline some- 

 times occurs in A. Petersil Ehrb. by the formation of one, two, or 

 three lobes which proceed sometimes suddenly from the margin or 

 by a more gradual extension. 



2. Surface. — In only a few T species is the surface almost flat 

 from the centre to the border, e. g., A. exignus Witt., A. Barbadenxis 

 Ralfs ; in several it is flat to the zone of the processes, but beyond 

 this slopes distinctly to the border, e.g., A. Comber i Arnott, A. 

 Beevaia Johnson ; in many there is an elevated band either at or 

 within the zone of the processes, and this may be associated with 

 the presence of inflated areas along the primary rays, e.g., A. 

 decorm Grev., A. Archawjehkianw Witt., A* superbus Kitton, <fcc, or 

 it may not, as in A. 6 dlittiayivs Norman, A. vumjaritacem Rail's, 



1. Lahusari Witt., &c. ; distinct inflations without a well-defined 

 narrow highest zone are normal in A. fomwsus Arnott, A. in hitus 

 Grev., A. mammosus Grev., A. Jatm hii Grove & Sturt, A. Carru- 

 thersicoms Kitton & Grove, A. quadrant Sch., etc. 



In some species the depression of the centre below the general 

 surface becomes considerable, and may either be accompanied by 



Journal of Botany.— Vol- 26- [April, 1888.] h 



