196 BRITISH PALEOZOIC PHYLLOCARIDA. 



already quoted above ; and the article " Crustacea," ' Encyclopaedia Britannica,' 

 vol. vi, 1877, p. 639, fig. 13; also Huxley's "Crayfish," 1888, pp. 56—60; 

 Rolleston's ' Forms of Life,' 2nd edit., by Mr. H. Jackson, 1888, pp. 181—184, 

 with full references to other observers ; and Howse's ' Atlas of Biology,' 

 pi. ix). 



In studying these little fossil teeth they should not lie crosswise, as in our 

 PI. XXVI, but the narrow and lowest (hinder) end of the tooth (pointing towards 

 the pylorus of the stomach) should be placed downwards, and the convexity will 

 be noticed on one side or the other. The cusps on the broad end (pointing 

 towards the cardiac part of the stomach) are thick and high, those at the narrow 

 end are low ; the hinder (pyloric) part of the tooth being more contracted and 

 depressed than the other (cardiac) half, as is markedly the case in the lobster 

 (Eomarus vulgaris) and crayfish (Astacus fiuviatilis). 



Our specimens are divisible into two groups : No. 1, those that have a con- 

 vexity on the left hand, as fig. 34, when the narrow or hinder (pyloric) end is 

 placed downwards ; and No. 2, those that are convex on the right, as in fig. 20, 

 when so placed. The anterior (cardiac) end of the tooth in PI. XXVI, fig. 34, 

 points to the right hand of the reader; and in fig. 20 it points to his left hand. 

 For convenience, and indeed according to their relative position in the lobster's 

 stomach, we term No. 1 group the sinistral, and No. 2 the dextral teeth. Both 

 kinds are present together in fig. 44, the sinistral tooth lying upon and against its 

 dextral fellow. 



These, looking like molar teeth of some sort of the higher animals, have a 

 crown consisting of six or seven cusps on a slightly curved solid basis, thickest in 

 the middle. Four of the cusps on the front (cardiac) half are thick and promi- 

 nent; but on the other (pyloric) half, two or three seem to have been worn down, 

 leaving a more or less flattened or hollow surface : for instance, figs. 18, 23, 25, 

 and 34 of the left teeth ; and figs. 20, 21, 40, and 43 of the right teeth. 



There is another tooth in the lobster, namely, the narrow, blunt, hook-like, 

 overhanging median tooth (bifid in the crayfish), which works between the hinder 

 flattened surfaces of the two side teeth below. 



"We have no evidence, however, in Dithyrocaris of the upper intervening median 

 tooth, such as is associated with a similar flatness in the hinder part of the side 

 teeth in the stomach of the lobster and crayfish. 



The illustrations of PI. XXVI were made before we had assorted the teeth in 

 the manner above mentioned. We now find that they fall into the following 

 arrangement, as rights and lefts : 



