OPIilURELLA. 



155 



Forest Marble, near Wollaston, I find the lateral plates pass ronnd and apparently grasp 

 the sides of the arms in a very distinct hook-like fashion. From the free border of these 

 lateral plates several short thorn-like spines project obliquely outwards ; between the central 

 and lateral plates two rows of apertures are formed by the mode these elements of the 

 rays are articulated together ; the arms are round, from three to four times the 

 length of the diameter of the body, and taper very gently from the disk to their termina- 

 tion. Since my plates were drawn I have seen a slab, collected from the Forest 

 Marble, near Weymouth, by my late friend the Rev, S. Cooke, F.G.S., on which two 

 specimens of this species are exposed lying on their under side and exposing the upper 

 surface of the disk and arms. The disk is composed of ten radial-plates, united in pairs, 

 and forming five heart-shaped shields, which cover the arms ; between them the disk is 

 depressed and undulated at the circumference. On its upper surface several thin small 

 scales are arranged in an imbricated manner and form a radiate pattern towards the centre. 

 The upper plates of the arms consist of two halves united by suture in the middle line ; 

 with the lateral plates they curve obliquely outwards clasping round the rays, and 

 terminate beneath the ventral plate, as shown in fig. 3 h and fig. 4. 



In fig. 3 a the under surface of the disk and arms of this species is shown of the 

 natural size. Fig. 3 3 is the same specimen, magnified three and a half diameters, and in 

 fig. 4 I have shown a portion of one of the arms with the spines articulated to the free 

 margin of the plates, and magnified six diameters. 



Affinities and Differences. — This Brittle-star resembles OpMoJepis Murravii. It is 

 altogether a more slender form, with a proportionately wider disk having larger and more 

 awl-shaped arms. In the structure of the ray-plates there is likewise a very important 

 difference, 0. Murravii having the encircling plates on the lateral parts of the rays, with 

 a total absence of the chain-like structure which distinguishes 0. Griesbachii. 



Locality and Stratic/rapJiical Position. — This Ophiurella was first discovered by the 

 late Rev. W. Griesbach in the beds of Forest Marble, near Wollaston, who generously 

 supplied me with several good specimens, one of which I have figured with details. 



I have a fair specimen from the Great Oolite of the Windrush Quarries, near 

 Sherborne, Gloucestershire, in a rock similar to that which yielded Solaster Moretoni, 

 Forb., so that both these extinct Sea-stars lived together in the same ocean. 



My late friend, Rev. S. Cooke, F.G.S., collected several thin slabs of Forest Marble, 

 near Weymouth, on which many beautiful specimens of this species he in bold relief, 

 and these specimens have enabled me to make out the structure of the upper surface of 

 the disk and rays : all the other specimens which I had previously examined displayed 

 only the ventral aspect of this Ophiurella ; which appears to be limited to the upper 

 portion of the Lower Oolitic Formation, as no evidence of its existence has been 

 found in the Murchisona, Soioerhyi, and Farlcinsoni zones, which faithfully represent the 

 three divisions of the Inferior Oolite in the English, French, and German Jurassic 

 strata. 



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