1881.] NATURAL SCIENCES OF PHILADELPHIA. 269 



spring off in exactly the same manner as the pinnules, and they 

 evidently were pinnule-like in their earlier form. 



The earliest Actinocrinidae known to us are found in the Upper 

 Silurian, but the species and even genera which already occur 

 there are so numerous, and show such variety of form, and some 

 of them appear to be so highly developed, that evidently the family 

 had been represented at a much earlier epoch. It is possible that 

 Schizocrinus Hall of the Lower Silurian, which is imperfectly 

 known, should be referred to the Actinocrinida, but it may have 

 underbasals. In the earlier representatives of this family, the 

 underbasals form the criterion by which alone the Actinocrinidfe 

 and Rhodocrinidfe can be distinguished, and as these plates in the 

 earlier types are very minute, it is often exceedingly difficult to 

 make the separation. In Glyptocrinus the underbasals may per- 

 haps be absent in some species, but when visible they are exceed- 

 ingly rudimentary. Species without them might be referred, 

 almost with the same proprietj' to the ActinocrinidjB, and indeed, 

 they have a remarkably close resemblance to species of IMocrinus 

 and Mariacrinus yvith. four basal plates. A similar relation exists 

 between Dimerocrinus and Stelidiocriniis, Glyptaster' and Perie- 

 chocrinus, which can be distinguished only by the underbasals. 



In Stelidwcrinus and Melocrinus, we recognize representatives 

 of two of the four divisions of the Actinocrinidse which occur in 

 the Upper Silurian. The two differ essentially in the relative 

 size of their body, and in the number and distribution of the 

 plates in the calj'x, but agree in the arrangement of their anal 

 area. Carpocrinus and Periechocrinus, which belong to the same 

 geological age, are separated by the very same characters, the 

 former agreeing closely with Stelidiocriniis, the latter with 

 Melocrinus^ but both are readily distinguished by having a special 

 anal plate in line with the first radials. 



The Stelidiocriniles disappear in the Upper Silurian, where 

 they are first known. The Periechocrimtes and Agaricocrinites 

 survived to the Subcarboniferous, the former to the Burlington, 

 the latter to the Keokuk epoch. The Melocrinites became extinct 

 in the Devonian. The Actinocrinites and Batocrinites are 

 restricted almost exclusively to the Subcarboniferous ; a few 

 aberrant forms are known from the Hamilton group. The 

 Actinocrinidse became altogether extinct after the age of the 

 Warsaw limestone. 



