THE FOSSIL CRINOID GENUS DOLATOCRINUS AND ITS ALLIES. 45 



pentagonal figure within the general basal concavity that is a con- 

 spicuous feature of the ornamentation. 



The tegmen is rather evenly convex to the base of the tube, and 

 its perfectly smooth and even surface, combined with the decanter 

 shape, produces a habitus that is highly distinctive. This charac- 

 teristic appearance is enhanced by the prominence of the pinnule 

 openings, wliich are large curved slits, conspicuously placed in the 

 open spaces between the arms, one at each side of every arm base, 

 and frequently two more in the interrays. The habitus thus indi- 

 cated is uniform among the type specimens of the several species 

 and also throughout a series of good specimens additional to the 

 types. 



The distinction chiefly relied upon by the authors in the descrip- 

 of these species is the number of arms, as to the sufficiency of which 

 they are quite frank. Under D. laguncula ^^ they say : 



In surface ornamentation and general form it resembles D. hammelli, but that species 

 has 16 arm openings, while this has only 14, which will readily distinguish the species. 



And under D. dissimilaris ^^ : 



In general form it resembles D. aplatus, but that species has 15 arms, while this has 

 13, so the arm formula alone will distinguish them. 



I have in hand for the study of this form the 7 figured types and 5 

 cotypes labeled with them, 9 other specimens from the Gurley col- 

 lection, and 16 from my own, making 37 in all. The range of varia- 

 tion in arms among them is from 11 to 17 in number, distributed as 

 follows: With 11 arms, 1; with 13 arms, 7; with 14 arms, 5; with 

 15 arms, 20; with 16 arms, 3; with 17 arms, 1. 



Thus more than half the specimens have 15 arms, and 95 per cent 

 of them have that number within one or two more or less; which 

 indicates that 15 is the normal number — 3 arms to the ray, as it is 

 almost without exception in the closely related D. triadactylus from 

 Michigan — the small deviations from the normal being due to spo- 

 radic increase or diminution irregularly in one or two rays. 



I have also examined the specimens with reference to the so-called 

 "ovarian apertures," which are so strongly featured in the descrip- 

 tions. As stated above, there are always a pair of them at each arm 

 base in this form, and frequently an additional pair between the main 

 ray divisions; but these differences are not uniform for the species as 

 described. For example, in D. hammelli, the "16-armed species," 

 the description says there are two apertures in each interradial area 

 and two separating the arms in each of the five rays, giving "20 of 

 these apertures;" yet one of the specimens selected by the authors 

 as a cotype, and also a 16-armed specimen of my own, have four 

 apertures between the rays, or about 40 in all. D. aplatus, the 



" Bull. 9, p. 52. " Idem, p. 55. 



