LOWER HELDERBERG ROCKS. 141 



usually, corresponding in number to the number of sides of the plate. Outside of 

 the nodes, the plates are marked by strong ridges, which diverge towards the sides 

 of the plates, and converge so as to meet on the lines from the angles to the centre 

 of the plate. The plates of the higher ranges gradually become free from nodes, and 

 are marked only by a central spine which often becomes very conspicuous, so that 

 the higher ranges of plates present a series of spiny crests. 



The lower parts of the arms preserved in the specimen appear to be composed of 

 a single series of plates; but it is probable, that, like many others, the succeeding 

 portions were composed of a double series of alternating and interlocking plates. 



Fig. 1. The body, and bases of the arms, with a portion of the column attached. 

 Fig. 2. Structure of the body to the base of the arms, as far as determined. 

 Fig. 3 - 14. Figures of separated plates from diflferent parts of the body, showing the form, 

 ridges, spines, etc. 



Fig. 15 — 18. Joints of the column, showing absence of nodes in some, and varying degrees 

 of development in others. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



Mariacrinas (Subgenus) Tecliiiocrinus andrewsi (n.s.). 



Plate LXXXVI. Fio. 1-4. 



Body hemispheric : base large. Surface ornamented by strongly elevated 

 ridges, which radiate towards the margins, and converge along lines 

 drawm from the angles to the centres of the plates. Basal plates much 

 wider than long. First radial plates large, about as long as wide, hexa- 

 gonal, with one heptagonal. Second radials smaller, and of similar form 

 to those below. Third radials pentagonal, supporting on their upper 

 sloping edges two arm-plates. Interradial plates, one hexagonal, resting 

 on the first radials and lying between the second radials, succeeded by 

 two hexagonal plates (or one hexagonal and one heptagonal ), and these 

 again by one large and one smaller plate. Brachial plates two at base, 

 six-sided, resting on the two upper sloping edges of the third radials : 

 each of the lower brachials supports two others upon the upper sloping 

 sides ; and from each of these originates an arm, making twenty al- 

 together. 



