C. Spence Bate — On ArchcBastaciis in the Lias. 309 



during fossilization, or it may be from the organ being hid or 

 reduced to a minimum value, as observed in the recent forms of 

 Willemoesia, or from its entire absence, as in Eryoniscus ; but the 

 presence of an orbital concavity determines that this ancient form 

 has retrograded from a species in which the organ was an important 

 feature. 



The first pair 'of antennae has three short joints to the peduncle, 

 and the remains of a pair of multiarticulate flagella to each. 



The second pair of antennas has very little of it preserved in the 

 specimen, but evidently carries an ovate scaphocerite ; that on the 

 right side is half lost, that on the left has the impression only ; the 

 rest of the organ is wanting on each side, except what I took to be 

 the impression of the distal joint of the peduncle, and the first of 

 the flagellum on the right side.-" 



On the right side the fil'st large chelate pereiopod is well defined, 

 although part of it exists only as an impression. That on the left 

 has been restored in outline from the right side, only a part being 

 preserved. 



All the other appendages are absent or hid beneath the body of the 

 animal, except those that go to form the RMpidura or tail fan. 



The outer plates are only determinable by the impression left in 

 the rock. They are broad, leaf-like, and rounded at the extremity, 

 without any sign of a diceresis or division in the outer plate, or a 

 tooth on the outer margin of the latter;^ the telson is broad, double- 

 ridged, and abruptly tapering. 



This species bears a generic resemblance with Polycheles crucifera 

 in the form of the carapace and P. MiXUeri and baccata in that of 

 the pleon, but differs from both in having no great dorsal ridge or 

 prominent teeth traversing the median line of the pleon, which in 

 this aspect more resembles that of some recent forms of Astacus. 



The fossil also differs from the recent Eryonidce in having a broad 

 open orbital notch, instead of a narrow cleft, in the dorsal surface of 

 the carapace, that is filled up with the upper surface of the base of 

 the rigidly attached ophthalmopod. 



The first pair of antennas, as far as I am able to interpret the 

 evidence at my disposal, has not the inner margin of the first joint 

 of the peduncle produced to an elevated ridge on the inner margin ; 

 this circumstance I think is largely due to the distance at which these 

 appendages are laterally separated from each other. 



The second or outer pair of antennas, if I have understood the 

 parts represented in the specimen correctly, approximates that in the 

 recent, and differs from that of Eryon in carrying a distinct scapho- 

 cerite at the base. It is true that Desmarest states that it is provided 

 with a large scale ; but he does not show it in his figure of the 

 animal, and although it has been so accepted by authors, I am not 



1 The Artist has since succeeded in making out distinctly the three short basal 

 joints of the second pair of antennse. — Edit. 



^ The Artist has indicated a division in the broad outer plate on the right side, 

 probably not clearly seen when the above description was penned by the author. — 

 Edit. 



