204 THE VOYAGE OF H.M.S. CHALLENGER. 



by a deep groove. The anterior is rather shorter than the posterior, and l)oth are pro- 

 longed some distance parallel with the dorsal edges, and form, as it were, lateral teeth 

 also. In the other valve (the left?) the edge is prominent on each side so as to fit in the 

 grooves in the opposite valve. Between the divergent cardinals under the umbones is 

 a triangular space which receives the internal ligament. 



Pythina arcuata, A. Adams, from the Philippine Islands, is a more inequilateral 

 species, with a less incurved ventral margin, and is difi"erent in the hinge, the posterior 

 tooth of the right valve being shorter than the front one, and neither laterally continued 

 as in Pythina paula. 



From the above description of the hinge of this species it will be seen that it would 

 be more correctly placed in the genus Montacuta. 



The dentition of the type of Pythina [Pythina deshayesiana, Hinds) is exactly that 

 of Kellia, and the fact of the shell being divaricately plicate does not in my opinion 

 entitle it to generic rank, but may perhaps be regarded of subgeneric importance. 



A few other species which have been described as PythinoB should certainly be placed 

 in the genus Kellia, such are Pythina mactroides, Hanley,^ and Pythina nuculoides, 

 lia,nlej^ = Erycina denticulata,Deshiiyes.^ Pythina cumingii, A. Adams,* does not quite 

 agree with Kellia in dentition, having two teeth on each side in the right valve and a 

 single one on each side in the left which fit in between those of the other valves. Two 

 other species, Pythina arcuata, A. Adams,' and Pythina triangidaris," A. Adams = 

 Mactra nucleus, (Conrad 1) Eeeve,' should be placed in the genus Montacuta and Pythina 

 stowei, Hutton,* although not absolutely identical as regards the hinge, sufticiently 

 resembles Pythina deshayesiana as to be placed in the same subgeneric section, having 

 similar divaricate sculpture. 



Montacuta angasi, n. sp. (PI. XII. figs. 2-26). 



Testa subaequilateralis, mediocriter convexa, ovalis, albida, vel dilutissime fuscescens, 

 vix nitida, incrementi lineis tenuibus striata. Latus anticum obtuse rotundatum, 

 posticum acutius ; margo ventris late, regulariter curvatus. Umbones fere mediani, 

 paulo supra marginem producti. Dentes laterales approximati, erecti. Pagina interna 

 subnitida, pallidissime fuscescens, radiatim obsolete substriata. Cicatrices profundse, 

 fere sequales, linea pallii simplice bene impressa junctse. 



This species is longish oval, rather more pointed behind than in front, obscurely 

 subtransparent, dirty whitish-brown, and sculptured with fine lines of growth. It is 



' Proc. Zool. Soc. Lond., 1856, p. 340. ■■' Loc. cit., p. 341. ^ Op. cit., 1855, p. 182. 



•• Op. cit., 185G, p. 47. ^ y^,_ ^ ,7^ i856, p. 47. " Op. cit., 1856, p. 47. 



' Coach. Icon., fig. 102 (gen. Madm). * Cat. Marine Moll. New Zeal., p. 76; Manual Moll. New Zeal, p. 157. 



