MORPHOLOGICAL PART. 79 
their mode of growth, and their gradual introduction in geological time. 
In the young dicyclic Crinoid, as we had occasion to observe especially well 
in the genus Platycrinus (Plate LXXIII. Fig. 10, and Plate LXXV. Fig. 11), 
the arms are uniserial throughout, their outlines waving, the plates decidedly 
wedge-shaped, the pinnules proportionally large and given off alternately as 
in true uniserial arms. In somewhat older specimens, the plates at the tips 
gradually interlock, and the new ones still forming at the distal end are 
strictly biserial. With advancing maturity the interlocking gradually ex- 
tends to the proximal ends, until finally in the adult Platycrinus the whole arm 
becomes biserial, except perhaps as to a few plates near the calyx, which 
permanently retain their larval condition. Similar modifications occurred 
in geological time. In the Lower Silurian the arms of monocyclic Camerata 
are uniserial, almost without exception. In the Niagara group and Wen- 
lock limestone, however, they rapidly change into biserial. It is very signi- 
ficant that among the species of that epoch we find as persistent characters 
all the phases through which the arms of the individual Crinoid pass in early 
life. This is well shown in the case of the Batocrinide.* The arms of 
Habrocrinus and Desmidocrinus are uniserial; but while the plates of the 
former are always rectangular, those of the latter in some species are de- 
cidedly cuneate. The same modifications can be observed among the arms 
of Patelliocrinus and Stelidioerinus, but in some of their species the cuneate 
plates already begin to turn into biserial by interlocking. We thus find in 
the same genus, and almost contemporaneously, all the variations from uni- 
serial arms to biserial; and, what is most significant, the arms of all Devo- 
nian and later Batocrinidex are strictly biserial. Turning to the Platycrinide, 
we find that the Upper Silurian Cordylocrinus has uniserial arms, formed of 
rectangular or cuneate joints, while in Marswpiocrinus, Culicocrinus, and Platy- 
crinus they are biserial. We may note also the case of the Hexacrinida, in 
which the development of the arms took place at a later period. The arms 
of the Devonian genus Heracrinus are uniserial, and also those of the earlier 
species of Dichocrinus. The plates of the latter are rectangular in all Kinder- 
hook species, and also in about half of those from the Burlington and Keokuk 
groups; in most of the others they are wedge-shaped, and in a few of them 
the arms fairly enter the interlocking stage. All species, however, of the 
Kaskaskia group have biserial arms, and likewise the contemporaneous 
Talarocrinus and Pterotocrinus. Among the Melocrinide, Glyptocrinus and 
* We have separated the Batocrinide from the Actinocrinide, referring to them only those genera in 
which the anal plate is followed by three interbrachial pieces, instead of two as in the latter family. 
