TEOPIDASTER. 



103 



from below it resembles an Astropecten. The rays are rather short, about equal in length 

 to the breadth of the disc, ovato-triangular. The upper surface of the rays and disc is 

 covered with short, obtuse, simple spines, which, on the sides of the rays, are ranged in 

 oblique rows of about five or six in each row. Placed rather laterally on the disc, is seen, 

 though obscurely, a madreporiform plate, and I think I can perceive indications of an 

 anal pore. Down the centre of each ray runs a keel composed of two rows of squamous 

 plates, somewhat quadrate in form, but produced at their anterior and inner angles (fig. 2 a). 

 This kind of keel, or mid-rib, is not present in an}'' star-fish, recent or fossil, with which I 

 am acquainted, and resembles in form and structure the tiling of the crest of the roof of a 

 house. It must be regarded as a peculiarity of generic value. The rays themselves appear 

 to have been very flexible, and not much liable to injury. Their extremities are rather 

 pointed. On the under surface their centres are occupied by rather broad, lanceolate, 

 ambulacral grooves running from the mouth (fig. 2 a) ; that orifice is somewhat contracted 

 by the encroachment of the large, twin, triangular plates (fig. 3), with punctated surfaces, 

 which occupy the angles formed by the junction of the bases of the rays. The ambu- 

 lacral ossicula (fig. 3 c) are oblong, rather broad, strongly geniculated in the centre at their 

 anterior margins, and denticulated by about five crenations at the edge which borders the 

 ambulacral sulcus. The margins of the under surfaces of the rays are bordered by trans- 

 verse, oblong, rather narrow plates (fig. 2 c), each bearing a crest at its anterior border, 

 indented by the sockets of eight or more rather short cylindrical spines, which have rugose 

 surfaces (fig. 2 e). It is these crested marginal plates, with their rows of spines, which give 

 the under surface of this star-fish so much the aspect of an Astropecten. 



Affinities and differences. — The atfinities of Tropidaster have been so fully pointed out 

 in the preceding description that it seems mmecessary to enter into more details on this 

 branch of the subject. Since the specimen originally described by Professor Porbes was 

 found, much larger and finer examples were discovered by my friend, Mr. Gavey, in the 

 same locality ; the best of these fossils I have figured in PI. Ill, fig. 3, whicli shows very 

 distinctly the large, prominent, twin, triangular plates around the mouth. 



Locality and Stratigrapliical Position. — This remarkable Star-fish was discovered by 

 G. E. Gavey, Esq., E.G.S., in the Middle Lias at Mickelton Tunnel, near Chipping Campden, 

 Gloucestershire, where it was associated with Cidaris Bdwardsii, Wr., Ur aster Gavey i, 

 Porb., Palceocoma Gaveyi, y^w, ondi Pentacriniis robustus, Wr., together with a series of 

 Mollusca, characteristic of the zone of Ammonites capricornus, as Ammonites capricornus, 

 Schloth., Ammonites Henleyi, Sow., and the usual species of Conchifera found interred 

 with these Cephalopods. 



