iqo8.] R. E. lyLOYD : Bathynomus giganteus. 93 



limbs of the V are continued forward, on either side of the œsophagus as two 

 chitinous rods, each of which passes over the optic nerve and the nerve to the second 

 antenna of the same side, and, after becoming fan shaped, joins the inner 

 surface of the carapace along an oblique line close above the fossa of the first 

 antenna. This line of junction is clearly indicated externally by a faint groove 

 which is distinct in the largest specimens; immediately around this groove, the 

 minute pits elsewhere present on the carapace, are wanting, and the carapace has a 

 lighter colour. This feature is shown in pi. iii, fig. 2, of Bouvier's monograph as a 

 somewhat curved linear area of a lighter tint situated 2 or 3 mm. above the antennary 

 fossae 



The posterior portions of the sternal alae plates are shaped some^^-hat like the 

 wings of a butterfly, and occupy a position half-way between the vertical and the 

 horizontal. Each is composed of two laminae of chitin, between which is a certain 

 amount of fatty material. Their lower or outer surfaces give attachment to muscle 

 fibres, which are inserted into the first and second maxillae and the maxilliped. 



lyCt us pass now to a consideration of the tergal alae. These are a pair of rigid 

 chitinous structures, the principal function of which is to give attachment to the 

 powerful mandibular muscles : arising on either hand from the roof and side walls of 

 the cephalon, they occupy a position half-way between the vertical and horizontal ; 

 so that they present nearly the same appearance when viewed from above, as from 

 behind. The upper or posterior surface of each of these is smooth and convex, the 

 lower or anterior surface is concave and forms the posterior limit of a large fossa. 

 This fossa is covered by the lateral part of the dorsum of the cephalon ; it is bounded 

 externally by the side wall of the same including the eye, internally it is limited by 

 the chitinous rods which lie on either side of the œsophagus. The space limited by 

 these boundaries measures 23 by 12 mm in its greatest length and breadth, and is 

 wholly occupied by the powerful mandibular muscle, part of the fibres of which are 

 in close proximity to the inner side of the eye. 



The line of attachment of the tergal alae to the carapace is of interest. The 

 upper surface of the cephalon of Bathynomus , and some other Isopods such as the 

 CirolanidcB , shows two deep grooves on either side not far from its posterior edge. 

 These grooves are usually considered to have resulted from the union of the first 

 thoracic segment with the fifth cephalic ; a union which, from the presence of the 

 maxilliped among the appendages of the cephalon, we know to have occurred. These 

 grooves are continued downwards on the side wall of the cephalon and end below in a 

 deep pit, placed to the outer side of the interval between the mandible and first 

 maxilla. This pit, which is seen in fig. 3, is also clearly shown in pi. iv, fig. 9, of 

 Bouvier's monograph. Internal examination of the head shows that the tergal alae 

 arise in their whole length from these grooves, and they appear as though formed by 

 invagination of the cuticle along their line. If these grooves indicate the union of 

 the fifth cephalic with the first thoracic segment, we should expect them to end be- 

 low in the interval between the second maxilla and the maxilliped. 



The internal skeleton of the fore part of the head shows some interesting features. 



