FOSSIL INPUSOHIA. 161 



two of the same spots will be seen, one on eacli side ; tliey are of tri- 

 angular figure, and appear to be thicker parts of the shell, described 

 as holes by Ehrenberg. Four smaller triangular spots may be observed 

 m the same lorica, one being situated at each corner; these also have 

 been considered as openings by Ehrenberg : their length varies con- 

 siderably; some exceed 1-lOOth, whilst others are even smaller than 

 1-lOOOthof aninch. 



Isthmia Unervis (No. 5) is found attached to sea-weed. It is in 

 three parts, and of a trapezoid shape. The centre part appears like a 

 band passing over, and is bounded by broad straight lines. Its outer 

 surface is covered with a network of rounded reticulations, arranged in 

 parallel lines. Among the most remarkable are Amphitetras antedilu- 

 viana ; they are of a cubical or box-like figure, and consist of three 

 portions, the one in the centre being in the form of a band, as shown 

 at Plate II. No. 8, and the two lateral ones having four slightly-pro- 

 jecting angles, with an opening into each. When viewed in detached 

 pieces, the central one is like a box, and the two lateral portions 

 resemble the cover and bottom. The former may be readily known, 

 as consisting merely of a square frame-work with striated sides ; but 

 both the latter are marked with radiating reticulations. When recent, 

 they are found in zigzag chains, from their cohering only by alternate 

 angles. In some instances, as in Biddulphia and Isthmia, two young 

 specimens may be found within an old one. 



Coccorms is oval, and is marked with eight or ten lines proceeding 

 from the inner margin to the centre; between which are dotted fur- 

 rows, which leave a small elder spot in the centre. (Plate II. No. 3.) 

 Oampylodiscus Glypeus is oval, and curved in opposite ways at the 

 long and short diameters. On the margin there are two series of dots, 

 sometimes joined ; and on the oval centre there are also dots about the 

 margin, while the middle is nearly plain. (Plate II. No. 7.) 



Actinocydus has a round bivalve flinty case, with numerous cells 

 formed by radiating partitions. Sometimes only every alternate cell 

 is on the same plane. The specimen in the Museum of the College of 

 Surgeons is exquisite in its markings ; it was found in some Bermuda 

 earth, and has a beautifully raised margin, and a five-rayed star in the 

 centre ; the number of cells is ten, five being on one plane and five on 

 another. One set has the usual hexagonal reticulations crossed with 

 diagonal lines, the other has the same lines, with a much smaller series 

 of triangular reticulations, so disposed that they appear to form with 

 each other parts of minute circles. One of the valves of this specimen 

 is represented in Plate II. No. 2. 



M 



