494 RADIATA. CRINOIDiE. Pentacrinus. 



foreign bodies ; from a depression in the centre of this plate arises the filiform 

 column, slightly incrassated towards the summit, and capable of bending or 

 twisting, composed of about twenty-four joints ; tbe external form and inter- 

 nal structure of the column not ascertained. Axillary side arms in a single 

 row around the last joint of the column, corresponding with each of the costals, 

 revolute, composed of about ten joints, the terminal one hooked. Plates of the 

 pelvis too obscure to be determined. The first costals are thick, wedge-shaped, 

 broadest upwards, the distal extremity emarginate, and supporting a second 

 costal of a narrow subconical form, truncated and emarginate where it joins 

 the scapula. The summit of the scapula is angular, having a sloping face on 

 each side, for the attachment of the first arm joints. The arms are ten in 

 number, and being supported on scapulae greatly narrower than the first cos- 

 tals, are rather remote at their base. The arms consist of about twenty -four 

 solid joints, tapering towards the extremity, simple in youth, but with age 

 and size dividing and subdividing ; along the side a row of dark spots ; each 

 arm supports on each side, in an alternate order, a row of annulated cylin- 

 drical, flexible, fleshy processes, covered with capitate filaments. The arms 

 in these respects resemble those of the Comatula, in which I have observed a 

 similar row of brown tubercles with a granular surface, a fleshy production 

 on the ventral margin of the arm, broadest at the middle, supporting cylin- 

 drical processes, fringed with smaller filaments, ending in an expanded 

 sucker-like Extremity. Within the costals, the body is subglobular, in the 

 centre of which is the mouth, formed by five petal-like valves, within which 

 are several soft tentacula ; without the valves is an opening with a tubular 

 extensile margin, supposed to be the vent. In the early stages of its growth 

 neither column nor arms are visible, the body appearing like a little club, fixed 

 by an expanded basis, and giving exit, at its apex, to a few pellucid tentacula. 

 The existence of the second aperture in the body of the genus Comatula, which 

 Mr Thompson, in the interesting paper from which we have extracted the de- 

 scription of this species, supposes not to have been previously observed, was 

 known to Mr Adams thirty years ago, and described by him in the Linnean 

 Transactions published in "1800. In the Annals of Philosophy for Nov. 1806, 

 Mr Gray likewise announced the existence of this second aperture, without 

 being aware of its previous detection. If£future observations, on better pre- 

 served specimens, shall confirm the results of Mr Gray's examination of the 

 recent Pentacrinus caput Medusa:, and demonstrate that it has but one open- 

 ing to its alimentary canal, it will be necessary to constitute this species, dis- 

 covered by Mr Thompson, into a new genus, which may be termed Hiber- 

 nula (the forerunner we hope of many important additions to our Fauna, 

 when the shores of Ireland shall have been more extensively explored), and 

 by characterising the family Comatulad^e, as possessing, besides numerous 

 other common properties, an alimentary canal with two openings, it will be 

 naturally divided into two sections, the first containing the fixed Hibernula, 

 the second the free Comatula. 



EXTINCT SPECIES. 



1. P. Milleri Auxiliary side-arms round, in single series ; column smooth ; 



columnar joints generally alternately smaller and larger — P. Caput-medusae, 

 Mill. Crin. 56 — In Lias. 



2. P. basalitiformis Auxiliary side-arms round, in single series; column 



smooth ; columnar joints generally of more uniform thickness — Mill. Crin. 

 62. — Lias. 



3. P. tuberculatum Auxiliary side-arms round, in single series ; column 



tuberculated. — Mill. Crin. 64.— Lias. 



4. P. subangularis.— Auxiliary side-arms round, in a double series. Mill. 

 Crin. 59. — In Lias. 



