84 PROF. GROOM ON POLYPHYMA LAPWORTHI.  ([Feb. 1902, 
examination shows that the reticulate individuals differ from the 
rest in no other particular, and, moreover, reveals the fact that the 
reticulation is due to the presence of a ‘meshwork of intersecting 
cracks which are clearly of secondary origin. A similar reticulation 
is seen 1n some individuals of the associated Acrotreta. 
With the aid of the more perfect specimens I have now been able 
to reduce all the individuals, with the exception of one or two of 
the poorest specimens, to the single type, now to be deseribed. 
Il. Descriprron. 
The chief characteristics of Polyphyma Lapwortht are :—A sub- 
central, obliquely-directed reniform elevation; three tubercles near 
the dorsal margin; and an anterior subtriangular lobe (see the 
accompanying text-figure). 
Kestoration of Polyphyma Lapworthi, gen. et sp. nov. 
jie 
[ x 34 diam. ] 
The valves, when not flattened by pressure, are convex, and almost 
semicircular in outline, with a straight dorsal edge, and an almost 
uniformly and continuously curving ventral edge, the pestero-ventral 
margin alone being slightly protuberant in the larger specimens. 
The dorsal and ventral margins meet in a well-defined angle. The 
ventral border is strongly convex, except along the posterior margin 
