PEN 



[347] 



PEN 



gelatinous muscular integument. 

 From the circumstance of penta- 

 crinites abounding in the lias and 

 lower strata of the oolite, and dis- 

 appearing entirely in the upper- 

 most strata, geologists were dis- 

 posed to limit their existence to 

 certain periods, and to conclude 

 that the pentacrinite furnished the 

 remains of an extinct genus. Sub- 

 sequent discoveries, however, prove 

 that the pentacrinus does still exist, 

 and thus, probably, it may be with 

 many genera which, in ignorance, 

 geologists describe to be extinct, 

 merely because they have not met 

 with living or recent specimens. 

 The calcareous joints which com- 

 pose the fingers of the Pentacrinus 

 Europaeus, are capable of expansion 

 and contraction in all directions ; 

 now spreading outwards, like the 

 petals of an expanded flower, and 

 again rolled inwards towards the 

 mouth in the form of a closed bud. 

 These organs serve to seize, and 

 convey to the mouth, the food of 

 the animal. The number of bones 

 in each animal is computed at thirty 

 thousand. Dr. Buckland says that 

 the number of bones in the Briarean 

 Pentacrinite exceeded a hundred 

 and fifty thousand. The number 

 of bones in the fingers and tenta- 

 cula amount at least to a hundred 

 thousand, and fifty thousand more, 

 (which is considerably under the 

 real number), may be added for the 

 ossicula of the side arms. Each 

 bone requiring at least two fasciculi 

 of fibres, one for expansion, the 

 other for contraction, the Briarean 

 Pentacrinus must have had three 

 hundred thousand fasciculi of fibres 

 equivalent to muscles. 



" The pentacrinites differ consid- 

 erably," says Mantell, "in their 

 form and structure from the encri- 

 nites, the name of encrinite being 

 given to the species in which the 

 bones of the column are circular or 

 elliptical, that of pentacrinite to 



those which have an angular er 

 pentagonal stem. The lias of Lyme 

 Regis abounds in the remains of 

 these animals; and large slabs often 

 have the whole surface covered 

 with the plumose tentacula of pen- 

 tacrinites, converted into pyrites." 

 " There are instances," says 

 Miller, "of the column of this 

 genus having sometimes a tetra- 

 gonal, or hexagonal form, these I 

 consider as monstrous varieties." 



PENTADA'CTYL. (from TreWe, five, and 

 a/cTt/\o9, a finger, Gr.) Having 

 five fingers. Applied also to leaves. 



PE'NTAGON. (from TreWe, five, and 

 7oWa, an angle, Gr.) A figure 

 having five sides and five angles. 



PENTA'GONAL. Having five angles 

 and five sides ; quinquangular. 



PENTAGONA'STEE SE'MILUNATUS. A fos- 

 sil species of the stella marina. 



PENTAGOSTA'STEE EEGULA'EIS. A fossil 

 asterite, or species of the stella 

 marina. 



PE'NTAGYN. (from TreVre, five, and 

 yvvYj, a woman, Gr.) A plant 

 which has five pistils. 



PENTAHE'DEAL. Having five equal 

 sides. 



PENT IHE'D EON. (from TreW, five, 

 and edpa, a base, Gr.) A figure of 

 five equal sides. 



PENTA'MERTJS LIM'ESTONE. The name 

 assigned to a rock of the Helderberg 

 group. The Upper Pentamerus 

 beds, or May Hill sandstone, or 

 Upper Llandovery beds, attain a 

 thickness of 1000 feet, being a 

 formation of the Upper Silurian 

 period. 



PENTA'MERUS. A genus of fossil Bra- 

 chiopodous shells, described as 

 equilateral, inequivalve, one valve 

 being divided by a central septum 

 into two parts, the other by two 

 septa into three parts; the umbones- 

 are incurved and imperforate ; five 

 species of Pentamerus are found in 

 the Silurian system of rocks in 

 England and "Wales. Lycett. 



PENTA'NDEE. (from TreVre, five, and 



