182 Proceedings of the Royal Physical Society. 



Length of three joints of antennre, 1 "5 mm. 



Length of middle joint of antenna, 75 mm. 



Length of first eleven segments of body, 7 mm. 



Breadth of body across one of the larger segments (the eighth), 3 mm. 



Breadth of body across one of the smaller segments (the seventh), 2*5 mm. 



Length of leg attached to fourteenth segment, 4 '5 mm. 



Length of second segment of leg, 1'5 mm. 



No. II. 



Specimen for which fig. Ih is taken, fossilised sideways. 



Length of head, 5 mm. 



Breadth of head, 5 mm. 



Length of sixteen segments, 20 mm. 



Breadth of larger segments, 4 '5 mm. 



Breadth of smaller segments, 3*5 mm. 



Genus Archidesmus, nov. genus. 



Body long, fusiform, depressed, composed of very numerous 

 alternately larger and smaller free segments, the dorsal scutes 

 of tlie larger being produced into large rounded lateral lamellae, 

 the smaller somites being mere rings interposed between the 

 larger. Each somite bearing only one pair of six or seven 

 jointed depressed and flattened legs, which are produced into 

 spines at the articulations. 



The genus is named from the general likeness of these 

 Old Eed fossils to many members of the recent Poly- 

 desmidse, with which family they are very nearly aUied; 

 indeed, were it not that each segment were free in both 

 this genus and in Kampecaris, they would rank themselves 

 along with Craspedesoma, which genus they closely re- 

 semble in the character of the eyes, the antennae, and the 

 lateral lamellae. They might with propriety be erected into 

 a sub-family of the Polydesmidae, for which the name of 

 " The Archidesmidae " seems not inappropriate. 



Archidesmus Macnicoli, nov. spec. — PI. II., figs. 2-2a. 



Body composed of very numerous segments ; is flattened 

 horizontally, swelling gently from the head to about the 

 sixteenth segment ; from this it preserves its general breadth 

 or only tapers slightly till the twentieth segment from what 

 appears to be the tail is reached, whence it tapers rapidly 



