HIBBERTIA ORBICULARIS. 205 



These (Figs. 14 and 15) very rare representatives of a peculiar form occur 

 in a bard, bluish, calcareous shale, with numerous fragments of shells. Several 

 Posidonomi/ai lie on the back of one counterpart, and AiHcnIopecten abounds on the 

 bed-plane at the back of the other. Mr. John Smith collected them at the Linn, 

 Dairy, in the " Upper Limestone " series, twenty miles south-west of Glasgow. 



The presence of a peculiar hinge-structure is a striking feature, as in L. senig- 

 matica ; the pod-shape, though narrower and more graceful, — the stride more 

 strongly represented than in Fig. 13, — themesolateral, smaller than in Fig. 13, are 

 links between the two species, and supply distinctive characters for this one, 

 which may be termed occulta, for it is not yet defined in a clear light. 



The longitudinal crumple or fold of the dorsal edge may be hidden in the 

 middle line of Fig. 11, and underneach that of Fig. 12 a. It does not seem to be 

 traceable in any species of Dithyrocaris. It might possibly have some analogy to 

 the mid-dorsal, longitudinal, thin piece, which is part of and intermediate to the 

 junction-line in the valves of the Rhinocarulse? 



III. HiBBERTiA ORBICULARIS, Joiies and Woodward. Plate XXV, figs. 8a, Sh. 



HiBBERTiA ORBicuLAEis, Jones and Woodward, 1899. Geol. Mag., dec. 4, vol. vi, 



p. 390, pi. XV, fig. 4. 



This small Crustacean shield was presented to the British Museum many 

 years ago by our late friend Dr. John Millar, F.CS., of Bethnal House, Bethnal 

 Green. The specimen is embedded in a finely laminated, micaceous, non-cal- 

 careous shale (bluish-grey in section), and bears the label " Coal M., Burdiehouse." 

 It was at first believed to be one of Dr. Hibbert's types from Burdiehouse, but 

 nothing like this fossil was found to have been figured or mentioned in Dr. S 

 Hibbert's memoir - on the strata and fossils of Burdiehouse; therefore its occur- 

 rence there, as shown by its old label, is of great interest. 



It was described and figured in the ' Geological Magazine ' for September, 

 1899, as indicated above. 



Generic and SiJecijic Characters. — A nearly circular shield, 14 ram. broad by 

 15 mm. long, having an opening behind, which is bounded by two acute incurved 

 angular spines, forming the backward extremities of the concentric shield or 

 buckler-like test. The periphery of the concentric part of the test is turned up 

 sharply as a thin rim in the fossil, leaving a uniform narrow groove behind it, 



1 See Mr, J. M. Clarke's description of this " median plate " in ' The American Naturalist,' 

 September Ist, 1893, pp. 793— SOI, and the ' Fifteenth Annual Report of the State Geologist, New 

 York,' 1896, p. 69. 



- 'Trans. Roy. Soc. Edinburgh,' vol. xiii (1836), p. 280, plates. 



