Bibliographical Notices, 97 



description either of the genus or of the species. Each of the three 

 visible radials bears a single axillary brachial united to it by a 

 " gaping suture," and there is a similar gaping suture between the 

 first and second azygous plates, wiiich is a point of no little interest 

 if the latter really supports the " proboscis," as described by the 

 authors. But is their interpretation of this structure the correct 

 one ? This question is easily answered, for I have not the smallest 

 doubt that the supposed proboscis is merely an undivided ray, like 

 the anterior ray of Miller's own genus Missouricrlnus, which has 

 been noticed above. The first and second " azj^gous " plates in 

 Ahrotocrinus cijmosus have precisely the same form and general 

 relations as the anterior radial and the brachial which it bears in 

 Missourlcrinus admonitns. In each case the upper plate is quad- 

 rangular, separated externally from the lower one along its whole 

 width, and followed by a series of simple plates which are obviously 

 brachials in Missonricrimis. Is not this also the ease in Ahroto- 

 crinus ? Messrs. Miller and Gurley must forgive me for drawing 

 attention to this point; for if their interpretation of the structure 

 of Ahrotocrinus is correct, it represents a morphological type of 

 extreme interest in many ways, whereas if the supposed proboscis is 

 merely an undivided anterior ray like that of Missouricrinus, the 

 definition of Ahrotocrinus will need considerable alteration, even if 

 it still merits a generic position. 



It is well known that among the Poterioerinidae the anterior ray 

 is less developed than the others and is sometimes simple throughout. 

 Messrs. Miller and Gurley intimate that Ahrotocrinus probably 

 belongs to this family and that its arms are like those of Scapliio- 

 crinus. If they will refer to Hall's diagnosis of Scapliiocrinus 

 unicus from the Keokuk Group of Indiana *, they may read as 

 follows : — " Arms dividing on the second radial plate ; each division 

 bifurcating twice and rarely three times. The anterior ray has a 

 single arm, which is undivided throughout. This single arm is 

 a strongly distinctive character." The posterior side of the body 

 and arms of this species is represonted in fig. 5 on pi. sv. of the 

 fifth volume of the ' Illinois Geological Ueports ; ' while a reprint 

 of Hall's description, together with a good figure showing botli the 

 real " proboscis " or ventral tube and the undivided anterior ray, are 

 to be found in the sixth volume of the same series (p. 519, pi. xxix. 

 fig. 1). I cannot myself make out that Ahrotocrinus cyrnosus is 

 either generically or specifically distinct from Scajjhiocrinus unicus, 

 which occurs on the same horizon and at no very distant locality in 

 the same State. 



In his well-known memoir on " New Species of Crinoidea from 

 the Carboniferous Rocks of the Mississippi Yalley " f Hall gave a 

 full description of this species, which concluded as follows : — " This 

 species may be readily distinguished from any other of the genus by 

 the low, broad cup, the number and bifurcations of arms in the 



* Prelim. Descr. New Crinoidea, 18G1, p. 8. 



t Joum. Boston Soc. Nat. Hist. vol. vii. no. 2, ISGl (1808), p. 314. 



Ann. cib Mag. N. Hist. Ser. 6. Vol. viii. ^ 7 



