A STUDY IN CAECINOLOGY. 69 
efficiency as organs of natation o£ these broad blades borne at the end of 
slender and very mobile limbs is apparent. Evidently Raninoides is a 
strong swimmer and, a poor digger, and as such stands at the end of 
the series opposite from Ranina. As the posterior branchial orifices arc 
absent, one might expect to find special arrangements for an incurrent 
respiratory stream in the frontal region, but, though I have looked carefully 
for such, I have been quite unsuccessful. However, I will deal with 
this question further on. 
Lyreidus (fig. 4) exhibits so many resemblances to Raninoides that one 
can hardly doubt that the two are closely related, yet, as T shall show, they 
are clearly adapted to dift'erent conditions of life. In Lyreidus the abdomen 
is narrow ; its first three segments lie nearly in a straight line with the 
carapace, but the fourth is of peculiar shape, bears a strong median dorsal 
spine, and constitutes as it were the knuckle of a sharp ventral flexure. The 
sixth abdominal segment is long and narrow ; its posterior angles are pro- 
duced into small aliform processes which at first sight might be mistaken 
for vestigial uropods, but tliey are only processes of the tergum having on 
their ventral surfaces small concavities which, when the abdomen is flexed to 
its fullest extent, engage with small knobs on the two pterygoid processes 
extending backwards from the twelfth sternum. No such apparatus for 
locking the flexed abdomen to the sternum is seen in any other Haninid. 
The last pair of pereiopods are so slender that, like those of Raninoides, they 
may be described as filiform : they terminate in small flattened elliptical 
(lactyli. Correlated with the reduction of the abdomen and of the last pair 
of pereiopods is the absence of posterior branchial orifices. I have studied 
this point carefully and am certain that these orifices are non-existent in 
L. tridentatus, nor could I find any trace of them in the large specimen of 
L. clianneri in the Natural History Museum. As there are no posterior 
respiratory orifices the water conduits of the flanks are, as might be expected, 
absent. The epimera of the posterior thoracic somites are nearly flat and 
the edge of the branchiostegite is but slightly prominent and bare of setas. 
In the frontal and oral regions there are also many points of resemblance. 
Though the " front " is truncated and scarcely narrower than the broadest 
part of the carapace in Raninoides, the distance between the extra-orbital 
spines and the lateral spines shows that this region has undergone elongation. 
Further examination shows that it is the antennarj' somite that has been 
lengthened, for the antennai-y sternum, which has more or less the shape of an 
equilateral triangle in Notopiis and Notosceles, is an isosceles triangle in 
Raninoides. In Lyreidus the elongation of the antennary somite is carried 
to an extreme, and the front being narrowed, the fore part of the carapace is 
produced into the snout characteristic of the genus, the lateral spines being- 
situated far behind the orbits. As I shall show further on, the details of the 
