Z PROCEEDINGS OF THE NATIONAL MUSEUM vol.79 



suy * from southern China and Roddyia typa ^ from southern Penn- 

 sylvania. In addition to those mentioned, scattered fragments, as yet 

 undescribed, indicate the possible existence of still other types. The 

 great number of Silurian eurypterid species belonging to highly 

 varied genera indicates that their ancestry extended far back. Now 

 that we know merostomes among the oldest recorded fossils, we are 

 beginning to comprehend some details of this ancestry. 



BECKWITHIA, new genus 



Diagnosis. — Merostome with a large cephalothorax having a flat- 

 tened rim, ending in genal spines. Eyes rather large, possibly cres- 

 centic, situated well forward. Eight abdominal segments. Pleurae 

 bent down at a considerable angle at the middle and also rearward, 

 at an increasing angle from front to back. Pleural terminae bluntly 

 truncated. 



Body terminated by a spiniform telson with an extraordinarily 

 large basal plate. The spine arises anterior to the center and appar- 

 ently extended upward and backward for a distance equal to the 

 length of pygidium. 



Surface of cephalothorax strongly pustulose, remainder of body 

 more finely granulated. 



Gompariso)is. — Detailed comparisons with other merostomes are 

 incorporated in the specific description, but the consensus of opinion 

 among those who have seen this specimen is that it agrees most 

 closel}' with Aglaspis. 



Genotype. — Becktoithia typa., new species. ' 



Range. — Middle Cambrian, Marjum formation, central Utah. 



BECKWITHIA TYPA, new species 



PiATE 1, Figure 3 



Body. — General form ovate in outline, being nearly twice as long 

 as broad with the greatest width across the genal angles. In life 

 the animal must have been fairly highly arched, especially trans- 

 versely ; at least the wrinkling of the cephalon and the shape of the 

 thoracic segments indicate a moderate degree of compression suf- 

 fered by the fossil. 



Oephalofhorax. — Semicircular, nearly twice as broad as long. The 

 posterior margin forms a slight sigmoid curve on each side from the 

 center of the head to the genal angles. The lateral and anterior 

 margins, on the other hand, are regularly rounded, forming an out- 

 line that is but a little more curved than a semicircle. A flattened 

 border runs all around and projects into short genal spines, indicat- 

 ing the existence of a rather wide doublure. 



* Memoires du Service G^dlogique de rindochine, vol. 1, fasc. 1, p. 31, pi. 4, flg. 6, 1012. 

 s Resser, Proe. U. S. Nat. Mua., vol. 76, art. 9, p 13, pi. 2, flg. 5, 1929. 



