REVISION OF PALEOZOIC STELLEROIDEA. 185 



are not ambital. In the mid-length of the ray, however, these plates 

 greatly resemble the ambitals. Judging from the manner in which 

 they appear distally, their form and constant position beside and 

 outside the adambulacrals leads to the conclusion that they are the 

 inframarginals. Whether the inframarginals and interbrachial mar- 

 ginals are present at maturity cannot be determined. 



Formation and locality. Three specimens from the top of 'the Black 

 River limestone immediately beneath the Black River shale, or from 

 the Vanuxemia bed of the Black River (Mohawkian) group at Minne- 

 apolis, Minnesota. The specific name is after the distinguished 

 paleontologist, Dr. E. O. Ulrich, who found all the specimens. They 

 are in the United States National Museum. 



Remarks. This species is distinguished from other Ordovicic 

 Urasterellas by the few and widely separated plates of the radial 

 columns, and by the prominence of the supramarginal columns, 

 which in the other species are not distinguishable from the ambitals. 

 The convexity of the rays abactinally is also greater than in the 

 other species excepting U. Jiuxleyi, but that species is further dis- 

 tinguished in that it has smaller plates, there being from 7 to 10 in 

 5 mm., while in U. ulrichi there are only 5 to 6 in the same space. 



U. ulrichi and U. grandis attained a similar large size. The former, 

 however, is from a much lower horizon, has very convex rays abac- 

 tinally, two columns of larger central plates (supramarginals) instead 

 of one (radial), and the plates are smaller and therefore more 

 numerous. 



The young of this species are very interesting in that they show 

 distinctly the presence of large inframarginal and interbrachial 

 marginal plates in the axils. In U. pulchella none of these plates 

 have been seen in the axils, but the inframarginals are believed to 

 be present in the very small plates which are indistinguishable from 

 the ambital pieces situated along the greater length of the rays out- 

 side of the adambulacrals. The same appears to be true for U. grandis. 

 The young of U. ulrichi in its general structure reminds one much 

 of Mesopalseaster sJiqfferi, and it is probable that Mesopalseaster and 

 Urasterella are derived from the same stock, which also gave rise to 

 Hudsonaster. The latter retains throughout life the primitive plate 

 structure, while Mesopalseaster, and particularly Urasterella, develop 

 in various ways an abundance of accessory plates. 



The best preserved specimen of U. ulrichi (pi. 29, fig. 1) has two 

 nearly fully grown rays and three short stumps. One of these short rays 

 is so well preserved as to indicate that this condition is not due to poor 

 preservation, but is apparently a case of accidental loss of parts 

 during life. The wound has been healed, but no regeneration of lost 

 parts has taken place, as is so common in similar losses among the 

 living starfishes. 



Cat. No. 60612, U.S.N.M. 



