6 bulletin: museum of comparative zoology. 



broad, its width equaling or exceeding one half the total width of the 

 body, to the genus Bumastus. This practice will probably prevail, 

 for we now know that other illaenids beside Bumastus have ten seg- 

 ments in the thorax, and there is no illaenid known from which dorsal 

 furrows are absolutely absent. In view of the somewhat numerous 

 subdivisions of the illaenids, it may be well to reexamine the type- 

 species, Bumastus barriensis Murchison, in a little detail. Salter has 

 explained that the specimens figured by Murchison really do not 

 belong to this species, so that Ave are obliged to use Salter's figures of 

 the "Barr Trilobite." Fortunately the M. C. Z. collection contains 

 two plaster casts of the original specimen figured by Jukes in 1829 

 and later by Salter. The casts are rather carelessly made, but are in 

 general in fair agreement with Salter's figures. From these sources 

 may be derived the statement that the typical species of Bumastus 

 is a large Silui'ian illaenid with smooth, subequal cephalon and pygi- 

 dium, rounded, spineless genal angles, large eyes, situated near the 

 posterior margin of the cephalon, a very wide axial lobe, shallow dorsal 

 furrows, ten segments in the thorax, and no trace of an axial lobe on 

 the pygidium. The dorsal furrows on the cephalon are short, extend- 

 ing but little ahead of the eyes. The cephalon does not appear to 

 have any rim or concave depression, but the pygidium shows a slight 

 concavity, so that the profile of that member does not present a 

 smooth convex curve, but the curvature is reversed near the posterior 

 end of the pygidium. Both cephalon and pygidium are wider than 

 long. 



Archegonus Burmeister, 1843. 



Die organisation der trilobiten, 1843, p. 120, 121, pi. 5, f. 3. 



The type of this genus is Calyviene ? aequalis H. von Meyer, as this 

 was the only species cited by Burmeister in the first edition of his 

 Organisation der trilobiten. As the type is evidently not an illaenid, 

 but one of the Proetidae, the genus automatically disappears from the 

 family. 



Dysplanus Burmeister, 1843. 



Die organisation der trilobiten, 1843, p. 120. 



Type, Illaenus centrotus Dalman. 



The type of this genus is an illaenid with rather long, parabolic 

 head and abdomen-shields, spines at the genal angles, small eyes far 



