90 FOTEEIOCEINID^'E.— POTERIOCRnniS. 



Formation and Locality. 



The Mountain limestone, Hook Point, Wexford. 



In this species the lateral plates enclosing the digestive organs are much abbreviated, 

 and appear of considerable thickness. The rays, which are comparatively more massive 

 than in any other species of Poteriocrinus, articulate by the whole breadth of the radial 

 plates. Each of the five lower arms consists of two joints, the upper one being cunei- 

 form ; at this point these branch ofT into a pair, which generally consist of eight or nine 

 joints, but the number is not invariably constant, the upper one being also cuneiform 

 whatever the number may be. Here a second bifurcation takes place, the rays as is 

 usual, diminishing in thickness at every successive division. Their total number is at 

 least twenty ; but the specimens hitherto developed do not exhibit further divisions. 



The specimen from which our figure of this species is taken, was discovered by the 

 Authors in an outlier of the mountain limestone, known as the Hook Point, a rather 

 remarkable narrow tongue of land which stretches out from the Wexford coast, and partly 

 across the mouth of Waterford Haven. 



In the 5th Vol. of the Transactions of the Geological Society, plate 4, the several 

 figures numbered respectively, 4, 5, 6, and 7, appear to relate to animals belonging to 

 the genus Poteriocrinus, but the details as regards the arrangement of the plates forming 

 the calcareous skeleton, are not sufficiently clear to enable us to determine the species 

 to which they belong. 



The auxiliary side arms or claspers belonging to crinoids of the genera Poteriocrinus 

 and Actinocrinus, are arranged difTerently to those of Extracrinus and Pentacrinus. In 

 the Extracrinus Briareus, almost every one of the larger joints has its claspers, but in the 

 true Pentacrinus they only occur around the column at regular and distant intervals, the 

 intervening joints being destitute of lateral appendages. In the two first named genera, 

 the side arms occur only on the lower third portion of the column, and are so arranged, 

 that one or more lines passed spirally round the column, will bisect the point of attach- 

 ment of every side arm on it. 



