76 Part III. — Twenty-sixth Annual Report. 



Genus Ohondracanthus, De la Roche. 

 Ohondracanthus Williamsoni* sp. n. PI. III., figs. 8-17. 



In this species the body is depressed, and of an ovate form when seen 

 from above. Length about 7-5 millimeters, greatest width equal to about 

 half the length. The head, which is articulated to the thorax, is subquad- 

 rangular in its general outline, but has a shallow rounded projection on 

 each side, as shown in the figure (fig. 8). There is only a slight con- 

 striction between the cephalon and the thorax, and the neck is very short. 

 The thorax, which is considerably depressed, has the lateral margins 

 coarsely crenulated, or lobate ; there are about six rounded but somewhat 

 irregular projections or lobes on each side, the second and the last three 

 being more prominent than the others. The posterior end of the thorax 

 also terminates in a narrowly rounded median lobe, as shown in the 

 figure (fig 8). The abdomen is much reduced in size and of a rounded 

 form. 



The antennules are small, uniarticulated, and rather rudimentary in 

 structure (fig 10). 



The antennae are moderately large, and each consists of a stout basal 

 part, to which is articulated a strong terminal hooked spine, the whole 

 appendage thus forming a powerful grasping organ (fig. 11). 



The mandibles are similar to those observed in other species of 

 Ohondracanthus : they consist each of a very short basal joint, and a 

 terminal and broadly falciform masticatory part, both edges of which 

 are finely serrated (fig. 12). The dilated appendage, armed with two 

 short stout spines, situated at the base of the mandible, represents the 

 maxilla (see fig. 12). 



Both pairs of maxillipeds are very small. The first pair are very 

 similar in structure to those of Ohondracanthus comutus ; each consists of 

 a moderately stout base and terminating iu a straight claw-like spine, finely 

 setose on the inner edge (fig. 13). 



The second pair are rather larger than the first ; they each consist of 

 two joints of nearly equal width, but differing slightly in length, and 

 armed at the apex with a short and claw-like spine and a small rounded 

 process, as shown in the drawing (fig. 14). 



There are only two pairs of thoracic limbs ; both pairs are rudimentary, 

 and are also similar in structure Each limb consists of a short and 

 broad basal part, gibbous on each side, which bears a small oblong process, 

 separated from the basal part by a narrow constriction (figs. 15-16). 



The ovisacs were not very slender, and appeared to be of moderate 

 length, but they were more or less incomplete. 



Figure 9 represents a young female which, though resembling the adult 

 in having the lateral margins coarsely crenulated, differs in being pro- 

 portionally narrower. 



The male of this species is somewhat similar in form and structure to 

 that of Ohondracanthus comutus (O. F. Miiller). It is considerably 

 dilated in front, but tapers towards the posterior end (fig. 17). The 

 male is very small, being rather less than the -£$ of an inch iu length. 



The colour of this form is opaque white with a slight trace of red. 



Host Sebastes norcegicus (Ascan.), from Aberdeen Fish Market, 

 February 1908. Several specimens were found in the angles formed 

 by the gill cover and gill-arches, with the claw-like antennae of the 

 specimens firmly fixed in the tissues of the host. 



* After my colleague, Dr. H. C. Williamson, to whom I am indebted for this 

 and several other species. 



