1881.] NATURAL SCIENCES OF PHILADELPHIA. 221 



are composed of a double instead of single series of joints, and 

 bear pinnules. The difference from Mariacrinus thus actually 

 consists only in the much greater number of lateral arms, and 

 their being composed of a double series of plates. The increase 

 of arms evidently took place under the same conditions as in 

 Steganocrinus and Uiicladocrinus, the modifications in the arm 

 structure, due originally to individual growth, becoming fixed as 

 generic characters, and following a general rule, by which it seems 

 that the arms in all genera of the Sphieroidocrinida?, on passing 

 into the Devonian, change from single to double joints. 



Let us now consider some cases of Balocrinus in which an 

 increase of arms took place in the species under somewhat different 

 conditions. It has been shown b}- us (Proc. Acad. Xat. Sci. Phila., 

 18*78, p. 230) that Batocrinus Chnjsti, as a rule, has two arms 

 from each arm opening, differing thus from other species of that 

 genus. In the Acfinocrinites, and many other of the Spha.'roido- 

 crinidse, the arm openings are mere breaks in the bod}', and the 

 proximal arm joints consist of single plates, while in Batocrinus 

 they appear more like passages penetrating the test, and the arms 

 from their very base up are constructed of two series of pieces. 

 We have in our collections several specimens, which in ever}'- other 

 respect resemble B. Chrysti except that the}' have single arms. 

 They were obtained exclusively from the lower strata of the Upper 

 Burlington Limestone, the typical form of the species occurring 

 in greatest abundance in the upper la^'ers. The specimens with 

 double arms are generall}^ larger than the others, but we find them 

 also very small, thereby indicating that the modified arm structure 

 had passed beyond the stage of mere individual variation due to 

 growth, and became a permanent character of specific value 

 perhaps. B. Chrysti and its variety with single arms — for which 

 we propose the name B. Lovei — have twent}' arm openings, but 

 at the same time only twenty so called respiratory pores, which 

 are located, as usually in species with twent}' arms, above the 

 interradial and interaxillary areas. In both species the pores are 

 placed at like distances from the arm openings, wliich seems to 

 prove that the additional arm was given off from the opposite 

 side alternately from the pores. The arm starts from the first 

 free arm piece, which is changed into a bifurcating plate, but 

 without materially increasing its size. Toward the close of the 

 Burlington Limestone B. Chrysti underwent some changes, and 



