110 Mr. W. E. Collinge on an Abnormality 



XVII. — Note on an Interesting Abnormality in the Mandibular 

 Arch of Chi m sera monstrosa, L. By Walter E. 

 COLLINGE, M.Sc, F.L.S., &c, Research Fellow of the 

 University of St. Andrews, The Gatty Marine Laboratory, 

 St. Andrews. 



►Some little time ago, whilst examining- the visceral arches in 

 a specimen of Ohimcera monslrosa, I met with an interesting 

 abnormality in the form of a small bony substance situated 

 immediately posterior to the mandibular symphysis. More 

 recently, in examining further examples of this Cliimseroicl, 

 I had the good fortune to meet with a further but somewhat 

 similar abnormality. Having failed to find any mention of 

 a like occurrence in any of the accounts of the cranial anatomy 

 of this fish, a note upon the same may not be without interest. 

 In the first specimen the abnormality consists of the 

 presence of a small plate-like bone, not unlike in general 



Fio-. l. 



Outline figure of the mandible of C'hinuera monstrosa, showing the 

 position of the abnormal bone. 



ab.b., abnormal bone ; g.p., grinding-plate ; La., points of attachment 

 of ligaments ; m., mandible. 



shape the urohyal of some Teleostei. It is situated imme- 

 diately behind the lower posterior border of the mandibular 

 symphysis, being attached to each ramus by a short strong 

 ligament (tig. 2, lig.), which latter is inserted at the two 

 points marked l.a. in tig. 1. The bone is about 5 mm. long, 

 and readily stained with a special stain for osseous tissues. 

 Looked at from the ventral side, it is seen to consist of an 

 anterior triangular plate with a somewhat longer tail-like 

 portion posteriorly (fig. 2), the whole being confluent with a 

 dorsal expanded plate. In transverse section through the 

 anterior portion (fig. 2 a) the bone has the shape of a collar- 

 stud (tig. 3 a), whilst a section through the posterior part 



