16 PEOCEEDINGS OF THE ACADEMY OF [1887. 



Also the thoracic ribs terminate with long spines directed 

 obliquely backward. Each rib bears a deep depression in the 

 central i^art, which, commencing near the spinose peripheral end, 

 runs diagonally across the broader jiart of the ribs meeting the 

 rachis near the upper end of each annulation ; but jjrevious to that, 

 this depression sends off a side branch backward, which meets 

 the posterior part of the same annulus, leaving between the two 

 depressions a pointed triangular elevation in continuity Avith the 

 convex portion of the annuli ; the edges of the ribs project as 

 elevated rounded rims, broadest in the middle, and tapering 

 toward the rachis and toward the periphery, where the anterior 

 rim is seen to constitute the body of the projecting spines. 

 Embolimus rotundata, nov. sp. Pi. I, figs. 4 and 5. 



There is only one sj^ecimeu in the collection Avhich shows head, 

 thorax and pygidium in connection, but the movable cheeks are 

 missing. Other fragmentary specimens, however, exhibit the 

 head complete. The proportions in the size of the three partitions 

 of the body are : 



Head, 13 millimetres; thorax, 14 millimetres; joygidium, 9 

 millimetres. Nine thoracic ribs, as in the former species ; the 

 pygidium has six well-marked annulations and corresponding 

 pleural expansions, margin rounded, no si:inose projections. The 

 thoracic ribs are similar in configuration with those of the pre- 

 ceding sjjecies, but they terminate rather abruptly with short 

 pointed ends. 



The head likewise closely resembles the former species, with 

 the difference that the glabella expands considerably toward the 

 front end, while the glabella of the former is all its length of equal 

 width. The movable cheeks, observable in one of the specimens, 

 terminate backward in a long spine, reaching as far as the fourth 

 thoracic articulation. The two figures represent a fragment with 

 complete head and another incomplete specimen with the movable 

 cheeks missing. 

 Monocephalus Salter! ? Billings. Pi. I, fig. 6. 



I have, with some doubt, identified a small specimen found in 

 association with the others Avith the form described by Billings 

 under this name. The minuteness of the fossil, and its being 

 merely a cast without shell, prevents the observation of the more 

 delicate structural details, but the general appearance of Billings' 

 figure, as well as his description, are satisfactory proof to me that 



