INVERTEBRATES. . 171 



The following is the formula of this genus, as given by deKoninck and 

 Lehon, in their "Recherches Sui- les Crinoides Curb, de la Belgique:" 



"Basal pieces, 3, forming an expanded cup. 

 Radials, 2 . (one of which is large, and one small) -\-5. 

 An'als, 1 large, or 3 small. 

 Interradials, 1 + 4. 

 Number of arms, 10, 20, 30 or 35, according to the species." 



Column generally round near the body, more or less com- 

 pressed, and often tortuous below. Base pentagonal, depressed, 

 basin-shaped, or nearly flat, composed of one small quadrangu- 

 lar piece and two larger pentagonal pieces, all of which are 

 generally anchylosed. First primary radials large, and resting 

 on the base; second primary radial pieces small, trigonal or 

 pentagonal, and occupying a depression in the upper margin of 

 each first radial — usually supporting on their sloping sides the 

 first divisions of the arms. 



Arms ascending, or often, at first, extending nearly or quite 

 horizontally from the body, and bifurcating more or less fre- 

 quently, according to the species ; all the divisions being pro- 

 vided with tentacles. 



Dome composed of numerous irregular pieces, usually tuber- 

 cular, or sometimes forming spines; ventricose, more or less 

 depressed, or often produced in the form of an extended cylin- 

 drical proboscis. Opening lateral, or nearly central. 



As first proposed by Miller, this genus was badly defined, and it is manifest 

 that its author himself had no very clear ideas of its limits, since he also in- 

 cluded in it species of Pentremites, Say, Dichocinnus, Munster, and of his own 

 genus Actinocrinus. Later writers, however, have restricted it within far more 

 natural limits, and, as now generally understood, Miller's first species, P. Isevis, 

 seems to be regarded as the typical form of the genus. In this and the closely 



* It is worthy of note that although deKoninck and Lehon give two radials (one large 

 and one small) as the number, that their Fig. 1, PI. VI, of P. Isevis, Miller, shows 

 clearly 3 radials, one large and two small. As others figure and describe it as having 

 only two, this may be only an abnormal variety. It will be seen, however, that our 

 P. parvuhes, described on page 163 of Proceed. Aca.d. N. S., Philad., Aug., 1861, also 

 has three primary radials to each ray, while the arms are composed each of a single 

 series of pieces. 



